we threw her a surprise party because it was her birthday.

underline the cause and circle the effect

cause: it was her birthday
effect: we threw her a surprise party

is this correct?

I don't know what the teacher expects, but I think Daniel should use the whole clause -- "it was her birthday."

Right!

is the cause the whole: it was her birthday

can you help me

Yes, that is correct. The cause in this sentence is "it was her birthday" and the effect is "we threw her a surprise party." The cause is the reason or condition that leads to the effect. In this case, the fact that it was her birthday prompted or led to the action of throwing her a surprise party.