How long do you think ______ the Microsoft brings out a new product?
_____ has already been pointed out, grammar is not a set of dead rules.
.---There is a lot of smoke coming out of the teaching building.---Really? It _____be a fire, most probably.
–Did you find your new dictionary?---Yes. I found it ____I had left it.
–Will you ______me ____out this problem myself?---OK.
–When did the concert_____?---Oh, two days ago.