не..., а

1. The shape. Put не before a contrasted word or before the verb for a negated clause, then add a comma, а and the alternative. A later complement may carry the contrast even when не precedes the verb:

• Не е синьо, а зелено. (It is not blue but green.)
• Не днес, а утре. (Not today but tomorrow.)
• Не отиваме на кино, а на театър. (We are not going to the cinema but to the theatre.)

The verb is usually not repeated after а; the second part only names the new element.

2. The reverse order: а не. You can also state the right thing first and reject the other one afterwards: Искам чай, а не кафе. (I want tea, not coffee.)

3. Why not но? Но adds a fact that goes against what you might expect: Малък е, но е хубав. (It is small but nice.) Both facts stay true. With не ..., а the first idea is thrown out and replaced. In relaxed speech you will also hear ами in the same role: Не това, ами онова. (Not this one but that one.) In writing, а is the safe choice.

4. What can be corrected. Almost anything: a subject (Не аз, а той каза това.), a time, a place, an adjective or a whole action (Не спи, а чете. He is not sleeping but reading.). This builds on plain contrastive а, and it contrasts with не само ..., но и, which keeps the first element and adds a second one.