Sometimes we have words in our sentences that don't seem to mean anything, but they have to be there.
University of Michigan English Professor Anne Curzan says the word "do" is a good example.
"It does a lot of work in English, and it's almost invisible work, because it often doesn't mean anything," Curzan says, "but it has to be in the sentence to make it grammatical.
"The verb 'do' goes all the way back to old English, but its use as an auxiliary verb, as a helping verb, goes back only to the Renaissance. This would be in a sentence like, 'I do not know.'"