1、被动和主动（Passive vs. Active）
Tommy hit the ball.
The ball was hit by Tommy.（更长, 没必要）
Cigarette smoking has been shown to be dangerous.（不需要by sb.）
Halley's comet can be seen one every 76-years.
2、条件式（Conditional）
If......
Would......
描述可能发生的事情，如果某事发生。
If I had studied harder, I would have received an "A".
3、虚拟式（Subjunctive）, 表达请求，要求，希望，建议，假设。。。
The professor requested that the student complete（非completes） her assignment a day early.
I suggest he pay more attention in class, unless he wants to receive a "C" or lower.
I wish she were (非was) more punctual.
If I were president of the US, I would lower the unemployment rate.
If she were less hostile, she'd have more friends.
