But

But is nearly always a coordinationg conjunction. (Its' uses as a preposition and as an adverb are not common.)

The conjuntion but indicates a contrast between two ideas. Furthermore, if you want to find out but after not (only), please see the lesson double conjunction.

1. Conjunction

In a sentence "A but B", the information in B contrasts with the information in A. Let's see the table below:

A	Contrast	Y
Sam eats a lot,	but	never gets fat.
They used to have a cat,	but	now they have a dog.

A. But can link two whole clauses, as in the examples above, but sometimes we omit the first part of the second clause.
Ex:
1. She likes John, but (she) dislikes his opinions.
2. The car is old, but (it is) very valuable.

B. Notice the use of but not and but also at the beginning of the second clause or clause part:
Ex:
1. Sam has been to Florence, but not to Rome.
2. I enjoy reading novels, but not history books.
3. The government is increasing its chances of economic success, but also becoming very unpopular.

C. But can come at the beginning of a sentence. In conversation, you can use it to show that what you have to say contrasts with or disagrees with what someone else has said.
Ex:
A. This is the very best watch you can buy.
B. Yes, but how much does it cost?
A. It's just the thing for your husband.
B. But I'm not married!

Note: But sometimes links constrasting adjectives before a noun.
Ex: She gave away all her money. It was a generous but foolish thing to do.

2. Preposition

But as a preposition means "except, apart from".
Ex:
1. They've looked everywhere but in the kitchen.
2. Eat snails? I do anything but that!

3. Adverb

But as an adverb is rare. It means "just or only".
Ex:
1. He was but a baby when he first appeared on the stage.
2. We can but try. ("Things are not hopeless")
3. You can't but admire him. ("You cannot help it.")

I hope this article helps. If you have any question, please leave a comment below.