used for saying that what happens next is disappointing or unpleasant
I tore open the box, only to discover that some of the parts were missing.
used to give a reason why something is not possible or true
要不是/但是 （否定前句，然而相較於 only to，不一定是失望，有時也用於慶幸）
要不是: They would all have died except for [=but for] her and her quick thinking. (否定前句，指出「事實上」他們沒死，既然和事實相反，那就要用 would have 假設語氣)
但是: I want to go, except that I’m tired. (否定前句，指出他沒辦法去）
We would like go, except (that) it’s too far. (否定前句，指出我們沒辦法去）
only that = except that
只不過: He is correct, only that some details are omitted.
只可惜: This suit fits me well, except that the trousers are too long.
used to introduce a statement that indicates the only person or thing that is not included in or referred to by a previous statement 除了(連接詞)
這是 except 的正常用法，可以和 but 互換，作為連接詞，前面加什麼後面就加什麼。此外，他還能加個介詞來用。
He does nothing except [=but] complain. (動詞 except 動詞）
She didn’t leave the house all weekend, except to go to church.
Employees were not allowed to leave except in an emergency. [=unless there was an emergency]
Take no orders except from me. [=take orders only from me]
Everyone was invited except for [=but] me.