题目内容：

i don't know when i should leave 同意句
i don't know ________ __________ ___________(三空)
和
we must learn english well.(用necessary改写)
it's ______ _______ _______ ________ ________ english well

优质解答

i don't know when i should leave 同意句
i don't know when to leave.
we must learn english well.(用necessary改写)
it's necessary for us to learn english well