1.(1)be后的动词常用ing形式；(2)介词后面常用ing形式,如：be worried about doing sth.,be excited about doing sth.,be good/bad at doing sth.,be interested in doing sth.等；另外,有些动词后面的动词一定要用ing形式,如：enjoy,finish,practise,miss等.
2.一部分动词后面要搭配to do结构,如：ask sb.to do sth.,tell sb.to do sth.等；另外,to do还能表目的,如：She gives me a book to read.
3.(1)助动词后常用原形(现在完成时和过去完成时除外),如：He didn't call me./We won't go there by car.(2)祈使句中用动词原形,如：Open the door,please./Don't feed the animals.