So with ret eo da..., this construction is correct? : Ret eo din mont d'ar stal hiziv
yes that's right
Poent eo... essentially means "it is time to...", right?
Poent eo klask ma dilhad > It is time to gather/collect my clothes
yes. Klask is rather look for, search for (when you don't know where the thing is)
Oc'h evañ dour > here the verb form is the present participle
yes, as in "emaon oc'h evañ dour" = I am drinking water
En un evañ dour > here it is a gerund, right?
first of all, the particle is "en ur" (even before vowels, the "ur" part doesn't change like the article! and it always lenites the initial consonant, even z, unlike the article).
And then, "en ur evañ dour" means "at the same time as drinking water". "en ur" is mainly used to express the 2nd of 2 different actions: labourat en ur ganañ (working and singing at the same time), etc.
Gwelet em eus ur marc'h oc'h evañ dour > I saw a horse (that was) drinking water
Gwelet em eus ur marc'h en un evañ dour > I saw a horse while I was drinking water
exactly, except that it's "en ur evañ" as I said.
"Marc'h" means "stallion" in many places. Horse is usually "loen-kezeg" in Leon, and often "chao" or "jao" in Cornouaille
What are the differences with a-nevez, adarre and c'hoazh? I also came across dija--is that commonly used?
a-nevez often means "recently" (traditional songs often start with "me 'ya da ganañ deoc'h ur sonenn kompozet a-nevez", I'll sing you a song recently composed) and also in sentences like "an ti-se zo bet graet a-nevez", the house has been renovated (made a-new, literally). I don't remember having read or heard it often with the meaning "again".
Adarre means "once again". One can use "en-dro" instead.
C'hoazh means "yet" and "still".
N'eo ket erru c'hoazh = he's not arrived yet.
Emañ aze c'hoazh = he is still there.
"Dija" is rather used to say "already" when talking about your experience (an dra-se 'meus desket dija = I've already learnt that).