46.i don’t know much about historical events, so I can't help her with her history homework. → If I knew more about historical events, I could help her with her history homework.

47.This is the first time I have heard about the tram system in Hanoi. → I have never heard about the tram system in Hanoi before.

48.I had saved my document before the computer crashed. → After I had saved my document, the computer crashed.

49.I would prefer you to do your homework. → I would rather you did your homework.

50.I’m afraid our problems are only just beginning. → I’m afraid this is just the tip of the iceberg