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.'"
It's called the "dummy do."
The question, Curzan says, is why do we have to have a "do" there, and what does it mean?
"I don't think it means anything, but it has to be there in order to make the sentence negative. So you would say 'I know,' but if you want to make it negative, you put in the "not," and now, in modern English, you have to put in the 'do.'"
Curzan says this shows that we're not really streamlining our language.
"People will sometimes say to me, when I describing changes in the language – like some of the pronoun distinctions are getting lost between who and whom – and they'll say, 'Oh, well, English is just streamlining.'"
But Curzan says we have this example in the history of English where we made things much more complicated.
Hear the full discussion above.