You use but to introduce something which contrasts with what you have just said, or to introduce something which adds to what you have just said.

You use but when you are about to add something further in a discussion or to change the subject.

You use but after you have made an excuse or apologized for what you are just about to say.

You use but to introduce a reply to someone when you want to indicate surprise, disbelief, refusal, or protest.

But is used to mean 'except'.

But is used to mean 'only'.

You use buts in expressions like 'no buts' and 'ifs and buts' to refer to reasons someone gives for not doing something, especially when you do not think that they are good reasons.

You use cannot but ,could not but, and cannot help but when you want to emphasize that you believe something must be true and that there is no possibility of anything else being the case.

You use but for to introduce the only factor that causes a particular thing not to happen or not to be completely true.

You use but then or but then again before a remark which slightly contradicts what you have just said.

You use but then before a remark which suggests that what you have just said should not be regarded as surprising.