Learn Russian: Что это? Это не дом, а здание. - What is this? This is not a house, but a building.

In Russian, to say what something is not, you put the negative word не right in front of the word you are negating. Because we are specifically saying it is not a house, не goes directly before дом. Notice that Russian doesn't use a helper verb like "does not" or "is not".

Why does the sentence use а for "but" instead of но?

Russian has two common words for "but": а and но. We use а when we are making a direct contrast or correction, which almost always happens after a negative statement (like "not X, but Y"). Since we are correcting "not a house" with "a building", а is the exact word we need.

Why is there no word for "is" in Это не дом?

In the present tense, Russian drops the verb "to be" (am, is, are). You simply put the subject (Это, meaning "this") next to what it is or isn't (не дом, meaning "not house").

What is the difference between дом and здание?

дом usually means "house" or "home"—a residential building where people live. здание is the broader, more formal word for any "building" (like an office building, a factory, or a mall). This sentence is pointing out that the structure isn't a residential house, but some other type of building.