2.It took the workers three years to build the bridge.

take 必须是事或物作主语，后加时间，动词用to do 形式。

3.The book cost him one dollar.

cost 必须是事或物作主语，后见金钱。

4.He paid fifty yuan for a new dictionary.

pay 必须是人作主语，后加金钱，固定搭配pay for.

1．I think...句型,是一个含有宾语从句的主从复合句，意为"我想...，我认为..."。注意：后面的从句里如果有否定含义，往往把否定词由从句移到主句，这叫否定转移。如：I don,t think I,m going to stay at home .我想我明天不会呆在家里。像这样的词还有believe，suppose, expect。

2．if以引导的条件状语从句常译为"如果..."。注意：当主句是将来时的时候，从句要用一般现在时。如:If it rains tomorrow ，we,ll stay at home.

3.If we have it today, half the class won't come.

half the class=half of the class"一半的同学",

① half作名词，意为"一半，半个"复数形式是halves.half常用于"half a/an+名词"结构；表示"......个半......"时，可用"数词+名词+and a half"或"数词+and a half+名词（复数）"结构。②half 作指示代词，常与the,one's连用，构成"half the/one's +名词"，相当于"half of the/one's+名词"。作主语时，谓语动词的单复数要由half后接的名词决定。