1.The order came that the medical supplies (be sent) to Beijing for the H1N1 flu soon.
为什么不能用 must be sent?
2.Considering his ability,he (might have done) better.
为什么不能用 could have done?
3.Liza (may) well noe wang to go on the trip —— she hates traveling.
为什么不能用must?
4.I wanted to go the party,but my mom simply (would) not let me out so late at night.
为什么不能用could?
5.When asked anout their opinins abour the schoolmaster,mant teachers would prefer to see him step aside (in favor of) younger men.
这里的in favor of