用法：
1、(表示條件)如果 ,主句用將來時,if從句用現在時表示將來
He will come if you invite him.
如果你請他,他會來的.
2、(表示虛擬)假如,要是
時態：可以表示過去,現在和將來的情況.它的基本特點是時態退后.
a.同現在事實相反的假設.
句型 ：條件從句 主句
一般過去時 should( would) +動詞原形
If they were here,they would help you.
b.表示于過去事實相反的假設.
句型：條件從句 主句
過去完成時 should(would) have+ 過去分詞
If she had worked harder,she would have succeeded.
The rice would not have been burnt if you had been more careful.
If my lawyer had been here last Saturday,he would have prevented me from going.
If he had come yesterday,I should / would have told him about it.
含義：He did not come yesterday,so I did not tell him about it.
If he had not been ill and missed many classes,he would have made greater progress.
含義：He was ill and missed many lessons,so he did not make greater progress.
c.表示對將來的假想
句型：條件從句 主句
一般過去時 should+ 動詞原形
were+ 不定式 would + 動詞原形
should+ 動詞原形
If you succeeded,everything would be all right.
If you should succeed,everything would be all right.
If you were to succeed,everything would be all right.
3、是否 = whether 連接賓語從句
I wonder if she is ill.
不知她是否病了.