Learn Russian: Это не чай, а кофе.

In Russian, а is used for direct contrast or correction, like saying "not this, but rather that." Because the sentence corrects a misunderstanding (it is coffee, not tea), we use а. You would use но for a contrasting idea that doesn't replace the first one, like "It is coffee, but it is cold."

Where is the word for "is" in this sentence?

Russian drops the verb "to be" (is, am, are) in the present tense. The word Это means "this" or "this is." So, Это не чай literally translates word-for-word as "This not tea."

Why is не placed right before чай?

In Russian, you put the negative word не immediately before the exact word you want to negate. Since you are specifically negating the tea, it becomes не чай (not tea).

Is the comma before а required?

Yes! In Russian punctuation, you must always put a comma before the conjunctions а (and/but) and но (but). It is a strict rule that makes sentences very predictable to read.