I will post here some interesting and useful stuff about the language, its colloquialisms and such. I hope it will help you learners with better understanding how the spoken language actually works I shall update this once in a while when I have time, so feel free to interact.
Now I will begin with explaining the plural forms of masculine nouns, because there are more options and they can change the meaning of a word slightly (or even brutally, as you will see).
Now, masculine nouns in Czech fall under two major categories - animate (used for persons, peoples, animals and such) and inanimate (used for everything else). Those categories further break into six paradigms, which differ in conjugation patterns. Every masculine noun is placed under one conjugation paradigm, depending on the ending of the word in genitive case.
Those paradigms are:
pán (gentleman), gen. pána (animate)
hrad (castle), gen. hradu (inanimate) - there is actually a minor category within this paradigm, les (forest), which is 'lesa' in genitive
muž (man), gen. muže (animate)
stroj (machine), gen. stroje (inanimate)
předseda (chairman), gen. předsedy
soudce (judge), gen. soudce
Now, let's take a look at some plural endings, to see what's really going on here.
This is the most common paradigm of animate masculine nouns, and thus its plural system is most important to handle correctly. In most words, there are two options of marking a plural:
-ové/-é .. as in pánové (gentlemen)
-i .. as in páni (gentlemen)
Although nearly every word can be marked with both of these endings, usually there is only one correct way. Generally, you can assume that with taking the -ové/-é ending, you're being polite, and vice versa.
When you speak to a delegation of gentlemen, you're most likely to use the word 'pánové', as in 'dámy a pánové'* ('ladies and gentlemen'). The expression 'dámy a páni' is incorrect, albeit gramatically fine.
*little OT: choose wisely between saying 'dámy a pánové' or 'pánové a dámy' (lit. gentlemen and ladies) - the first form is official, the second somehow ironic and used f.e. between young people when addressing something to their own group, that is most likely nothing much like actual ladies and gentlemen 'Pánové a dámy, máme problém.' - 'Ladies and gentlemen, we have a problem.' Placing men before women is just rude, I guess.
When you hear the word 'páni', it most likely shows that the meaning of the word changed from default 'gentlemen' towards some colloquial thing, f.e. 'those who act like bosses' or something like that. Or, it's just an expression of surprise - either archaic or ironic one.
This can be extremely important when referring to nationalities or ethnic groups - and by extremely I mean extremely. Take this word:
Never ever use the word Židi, unless you want to sound like some inconsiderate redneck racist. 'V tomhle městě žijí Židé' means simply 'This town is populated by Jews', while 'V tomhle městě žijou Židi' would be translated like, say, 'This town is full of money-grabbing pricks.' So it's kinda clear what form you should use, I guess..
..nevertheless, this rule, although helpful in most cases, does not apply to every word:
Pražák (a person from Prague, col.) has only one plural form - Pražáci. Pražákové sounds just bizarrely polite and I can't imagine a person actually saying that (but you would be still understood). This is however more like a colloquial rule - the word Pražáci actually has something rude in it, because of the plural ending. The correct and polite form would be Pražané (Pražan in singular).
So I guess I can generalize a little here and say that by adding the -ové/-é suffix you would be right in most cases. Try avoiding the -i suffix unless you absolutely know what you are doing.
There are also two ways of making a plural:
The default one is -y, and it's also the only one actually used. So the situation is far more clearer that in the first paradigm.
hrady - castles
národy (sg. národ) - nations
So, what's that with -ové? This is actually kinda weird, but by placing the suffix -ové at the end of the word, you're automatically rendering it animate. This used to happen from time to time some 200 years ago, and almost only in poetry and such:
Národy povstaly. - The nations arose.
Národové povstali - The nations arose.
Dny se krátily. - The days were shortening.
Dnové se krátili. - The days were shortening.
The second version is brutally archaic and not even known to many Czech speakers, so just take it as an interesting detail and forget it (and notice how the verb ending changes - yea, this is gonna be real pain in the ass for you, as it is for the whole country. I and y are pronounced the same, so everyone hates this thing).
Another set of two endings here:
..and, as with that weird stuff about animate days and such, -ové is almost always omitted, so the default ending is -i, which is also automatically correct. You can say 'mužové' and it's no problem (although it does sound a bit stiff, so I use it only in academic writing), but with most words, the -ové suffix would sound just wrong.
Vietnamec - a Vietnamese person
Vietnamci - Vietnamese persons
Vietnamcové - Vietnamese persons
The 3rd version clearly says 'I'm either a foreigner or trying to be hyper correct here, because I think I can just take the rule of the first paradigm and apply it to the third one and that's the way this language works'. So, no.
So, have fun finding up the paradigm for each and every nationality or ethnic group before adding the proper plural ending
But, still, there are exceptions, because in Czech, exceptions are everywhere.
vítěz - winner
vítězové - winners
You won't hear the word 'vítězi', because it's just not right. Or it is, but it is the singular dative. 'Nuff trolled yet?
Yay, finally a paradigm with only one possible option!
No matter how polite, rude, casual or cute or whatever you want to sound, it's always -e here.
stroje - machines
Generally, the rules for inanimate nouns tend to be easier.
(I don't know if someone tried to mess up with 'strojové' centuries ago, but it's okay, because I don't wanna find out.)
Ah look, it's back.
předsedové - chairmen
dědové - granddads
I couldn't help this. The word 'dědové' might have two singular forms - 'děda' (paradigm 'předseda') and 'děd' (paradigm 'pán'), with the genitive of the latter being 'děda'. It means the same, but 'děd' is very bookish and therefore not commonly used. And great-granddad would be praděd, which is the name of a mountain. Czech is fun.
For the third time:
The distinction between those two endings is somehow optional here, both versions are correct, carry approximately same meanings and are in use, although -i is much more common.
soudcové - judges
soudci - judges
vynálezce - inventor
vynálezcové - inventors
vynálezci - inventors
You'll probably hear just the -i version, so you can go with this and you'll be always right.
Well, and now to add something spicier, to avoid sounding like a wikipedia article..
Well, so, every language has some 'parasitic words', that you repeat like all the time and stuff, you know. We were even trying to have an English conversation with this friend of mine, but without actually saying anything..
Yea, so, you know, I was like what, and she was all like whatever or shit, and I was like shit, really, you know, and she was like yea, whatever, you know, shit and stuff.
Felix Holzmann, a Czech comedian, made a hilarious sketch just about this. The funniest thing was that you could actually understand what was he trying to say, and it went on for some eight minutes. To cite Sheldon Cooper - it's funny, because it's true.
What I'm trying to say is that Czech is extremely prone to parasitic words. I, for example, can barely say a sentence without 'prostě'. This is an adverb which literally means 'simply', but it's kinda lost its meaning in most cases.
'Já to prostě udělám a prostě nevim, no.'
literally: Well, I will simply do it and I simply don't know.
what does it mean: I will do it and we will see.
Another cute word is 'vole'. This is in vocative case, which means it refers to addressing to someone/something, and thus it means 'You ox.' In Prague and Central Bohemia (and let's say the whole Bohemia as well, and the annoying preteen duckface people, and the internet), it's pronounced [voe:] (while sometimes written 'voe').
Needless to say, this is a very common way to talk between young people. In fact, it has become so common people don't realize that it actually could and did convey a meaning - I'd imagine this happened because you really don't come across many situations in which you'd need to talk to an ox. So, basically, everyone under 25 or something are oxen here in CZ.
Some people can create true art with this word:
Ty vole, bacha vole vůl vole, ty jseš ale vůl, vole.
literally: You ox, beware you ox, an ox, you ox, you are such an ox, you ox.
what does it mean: Be careful my good friend, there's an ox (presumably on the road).
Funny thing is, that the older you get, the less appropriate it sounds. When in your thirties, you should be avoiding 'vole' rapidly, because it just starts making you sound like a midlife-crisis-struggling manager, who just wants to stay young some more time. And, of course, it's still considered very rude among older people, and could cause very embarrassing situations, because although everyone's saying it, no-one's admitting they are.
I myself am speaking to a bunch of oxen everytime I'm out with friends, but outside direct citations, I don't use it at home in front of my parents. It can be somehow acceptable as a statement of surprise (Ty vole! - You ox!/I am really surprised!/Oh my god!/Wtf?), but never as a salutation, because that's reserved either for good friends, or for people +- the same age and social level. In school, f.e., you just pretend it doesn't exist, and try not to be heard, because teachers are no oxen. The important thing here is that when you conjugate the word anyhow (so it ends up in any other case than vocative) and than use it for referring to someone that's pissed you off, it (magically) gains back its true meaning and therefore becomes an insult - not the strongest one, but still.
'Náš učitel je hroznej vůl.' - Our teacher is a terrible ox.
'Co říkáš na našeho učitele, vole?' - What do you say about our teacher, you ox?
It loses all lexical meaning and becomes syntactically totally redundant habit of speech in the second case.
And there are more:
Tak určitě (Well, of course), famous for being overused by athletes as the ultimate beginning of every answer, and furthermore making the dumb-athletes-cliche alive and prospering;
Takže (Well/so), famous because every teacher you'd ever have in your life will try to destroy your habit to begin answers with that, but they'll never succeed;
Víš jak (You know how), or 'víš co' (you know what), the former pronounced [vi:žag] in Silesia. This is a perfect translation of English 'you know';
and so on.
So, when you meet a Czech (likely in a pub around midnight) and he'll tell you something like
'No, takže vole, nevim vole, prostě, víš jak'
all you need to know is that 'nevím' means 'I don't know'. He just doesn't know something.
'Well so you ox I don't know you ox simply you know how.'
And he's not even being rude!
Or maybe he is. Depends on the intonation, gestures, voice, and whether he's holding a weapon.