Why чаю, not чая, in "мы предлагаем гостям чашку чаю"?

Question

I understand the translation of Мы предлагаем гостям чашку чаю to be "We offer a cup of tea to guests".

But if this is so, shouldn't чай be in the genitive case (чая)? Why is it чаю?

Perhaps I'm interpreting the sentence incorrectly, or there is a nuanced grammar rule at play here.

Answer 1

(Translation of an answer in Russian)

This is an example of Partitive (частичного падежа). It is rarely used in Russian, usually with nouns that mean food or drinks.

Выпить воды (not all the water, but some water)

Поесть хлеба (some bread)

Выпить коньяку (some cognac)

Partitive usually matches Genitive case, but most masculine nouns like чай (tea), коньяк (cognac), бензинчик (gasoline), etc., have two forms of Partitive: чая/чаю, коньяка/коньяку, бензинчика/бензинчику. Both variants are correct, but second one (that matches Dative case) is colloquial.

So, in second case, in a sentence

Можно мне ещё чая/чаю?

Both variants are correct, because Partitive is used. However in case of "Я бы не отказался от чашки чая" the noun should be in Genitive case and so the correct form should be

Я бы не отказался от чашки чая.

Answer 2

Both "чашку чаю" and "чашку чая" are correct. The difference is that "чаю" is a spoken variant and "чая" is a stylistically neutral one.

Answer 3

Using the genitive in a partitive sense (kind of like "some..") is not that uncommon, but special partitive forms in -у are used less & less often these days. An American textbook in the 1970s ("Making Progress in Russian") listed about twenty words with -у partitive endings; most books now don't bother bringing it up, with the possible exception of чаю, which is still used, and народу (as in много народу). For a sense of this you can play with Google's ngram viewer: here's a comparison of много народу vs. много народа.