but · conjunction /bʌt/Full view
A1 used to introduce an added statement, usually something that is different from what you have said before
Dictionary examples:

You can invite Keith to the party, but please don't ask that friend of his.

She's very hard-working but not very imaginative.

She's not only a painter but also a writer.

I think it's true, but then, I'm no expert.

Learner example:

There are eight rooms but the best one is my bedroom, because there are a lot of books. (Key English Test; A2; Italian)

B1 used before you say why something did not happen or is not true
Dictionary example:

I was going to go to his party, but I was ill.

Learner example:

I'm really sorry I haven't written for so long, but I was very busy last week. (Preliminary English Test; B1; Italian)

but · preposition /bʌt/
B1 except
Dictionary examples:

Everyone but Andrew knows.

This car has been nothing but trouble - it's always breaking down!

Learner example:

I'm free all days of the week but Monday and Wednesday. (Key English Test; A2; Spanish)

Cambridge University Press