Here it means except (for) or apart from and we can substitute these prepositions for but in this sentence. We could also use bar which has the same meaning:

• All but / bar / except for / apart from two of the boys are coming with us.

but as conjunction

We usually think of but as a conjuction linking two contrastive sentences or clauses:

• They had very little money, but (they) always bought their children expensive presents.
• They were poor, but (they were) hardworking.
• My car is fifteen years old, but (it) still drives beautifully.
• I've been to Hong Kong but (I've never been to Shanghai) not to Shanghai.
• I sometimes swim in the North Sea, but (I) only (ever swim there) in July and August.
• I wanted to sign the contract there and then. But my husband insisted that we should read the small print first.
• These earrings would look really good on your wife! ~ But I'm not married!

In the first five examples, repeated information from the first clause can often be left out in the second clause.

But as preposition

We use but as an alternative to except (for), apart from and bar to introduce the only thing or person that the main part of the sentence does not include. It is often used after words such as everyone, nobody, anything, anywhere, all, no, none, any, every.

• I'll go anywhere for my holiday but / bar / except (for) Blackpool. I really hate it there.
• On holiday he eats nothing but / bar / apart from hamburgers and French fries.
• She took everything on holiday with her but / bar / apart from the kitchen sink.
• Everybody but / bar / the very young must carry their own belongings in a rucksack.
• I've marked all the essays but / bar / except (for) / apart from two.
• Nobody but / except (for) / bar Jessica would wear a mini-dress at a formal dinner

In a British court of law, a witness giving evidence is required to take the oath before he gives his testimony. He is required to say the following:

• I swear by Almightly God to tell the truth, the whole truth and nothing but the truth.

But if he has no religion, he says instead:

• I affirm that I will tell the truth, the whole truth and nothing but the truth.

Note the useful expressions next but one, last but one.

• They live in the house next but one to Mary. (i.e. two houses away from Mary)
• Is this the final candidate? ~ No, it's the last but one. (i.e. there are two more people to be interviewed)

but for

Note that but for as a preposition has a different meaning from but by itself. We can sometimes use it as an alternative to an
if-clause with a third conditional negative sentence, indicating what might have happened if other things had not happened. Compare the following:

• If it hadn't been for your generosity, I wouldn't have been able to go to America.
• But for your generosity, I wouldn't have been able to go to America.

• I would have been home in time for supper, if there had been no fog to delay me.
• I would have been home in time for supper but for the fog.

• But for his broken leg in the earlier part of the season, he might have been in the England team to play Poland last May.
• If he hadn't broken his leg in the earlier part in the season, he might have been in the England team to play Poland last May.

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