[I] sound is between [i] and [e]: https://en.m.wikipedia.org/wiki/Near-close_front_unrounded_vowel
There is also a special form of genitive "tea" which is spelled: чаю instead of чая.
These are tables I collected and collated because I found most tables to be too difficult to use, so I put them in a form I could understand and reference more easily.
For a whole bunch of tables, including the ones you ask for, look at: https://forum.duolingo.com/comment/29038061
There are links there to other tables I gathered and published. Feel free to copy, use, distribute, etc. I suppose I have a copy-right in them, but don't assert that right for stuff that is distributed free to others, although I'd appreciated a cite to my Duo username.
It is the preposition that functions as the verb "have" in English. If you've played video games, take it as the hotkey to access the inventory of the following noun:
- У smb. есть smth. - There is (есть) smth. in smb.'s inventory.
- У smb. нет smth. - There is no (нет) smth. in smb.'s inventory.
У literally means "by" or "near". The format of у...есть.... literally means "by/near [someone] is/exists [something]" which is translated quite freely (idiomatically) into "[someone] has [something].
у...нет... is the negative of that - "by/near [someone] (is) not [something], which is idiomatically translated into [someone] has no/does not have [something].
So here, у мамы нет чая literally means "by Mom is not tea" = "Mom has no tea".
2nd declension, m, (like m nouns ending in a consonant)
ns. Чай, gs. Ча́я, npl. Чаи́, gpl. Чаёв (https://en.m.wiktionary.org/wiki/чай#Russian)
Note re gpl.: "After a soft consonant, ё is written when stressed; е when unstressed." (https://en.m.wikipedia.org/wiki/Russian_declension)