用方框中所给动词的适当形式填空

learn  help    work     invite     do

You never know how much difficulty we had _________ out the problem.
I’ll go to the concert with you if all my homework ________ tomorrow afternoon.
The girl devoted all her time she had to _________ others.
He insists she ____________ to our party tomorrow.
The students ___________ 2000 English words by the end of last term.