To say “when”( in the evening, at night, this year) in Latin is used Abl-nocte, anno, or Abl+in-in pace, or Locativus-interdiu, noctu (if i`m not mistaken) and what about Greak language?
Should I use Dat [size=125]ἡμερᾳ, νυκτὶ[/size] or it is better Dat+[size=125]ἐν ‐ ἐν ἡμερᾳ, ἐν νυκτὶ[/size]or Gen [size=125]ἡμέρας, νυκτὸς[/size]? or maybe it is Acc?
Dative is for a specific point in time.
Accusative is for an extent of time (like “all day” or similar).
Smyth has a nice example contrasting these in section 1540:
ταύτην μὲν τὴν ἡμέραν αὐτοῦ ἔμειναν, τῇ δὲ ὑστεραίᾳ κτλ. throught that day they waited there, but on the following (day), etc.
For the genitive take a look at the Smyth grammar: it has several sections devoted to the different meanings, secc 1444-1447.
[size=134]τῇδε τῇ νυκτί[/size] dative without preposition = (precisely) this night
[size=134]ἐν νυκτί[/size] = Latin nocte
[size=134] νυκτός[/size] = Latin noctu
To Annis
Thank you for explanations, now everything seems much easier, just don`t have Smyth grammar, yet 
To Skylax,
You probanbly know almost all answers, just a real one on-line Greak language teacher 
By the way how to say ‘thank you’ in Greak?
1.[size=125]χάριν αποδίδωμι[/size]
2.[size=125]χάριν οἶδα[/size]
3.[size=125] χάριν ἔχω[/size]
4.something else
I don’t know what sort of internet connection you have, but we have a copy in PDF here at Textkit. It’s large, and might take a while to download.
As for “thank you” I have seen both 2 and 3.