助动词后面的动词第三人称不加-s。
助动词有两种，一种叫做时态助动词，一种叫做情态助动词。
一、时态助动词。
时态助动词有四种：be、have、do、will。
be动词用于构成进行时，它后面的动词要用
现在分词形式；它也可以用于构成
被动语态，后面的动词用
过去分词形式，如：
I'm learning German at home now.
He is playing the piano in his room.
The song is sung by Jolin.
I like the building which was designed by
Mr. Robertson.
助动词have用于构成完成时（包括完成进行时），它后面的动词要用过去分词形式，如：
I have learned English for one score years.
I have been learning English for one score years.
How long has he played the piano?
助动词do是为无助动词的动词提供助动词，是构成一般时的，它可以构成
一般现在时和一般过去时的否定句、
疑问句和部分倒装句，后面的动词用原形，如：
I don't speak English.
Did he make many mistakes on the test?
助动词will是构成
一般将来时和过去将来时的，后面的动词也要用原形，如：
Will it rain tomorrow?
I won't go on Friday. I'll go on Sunday.
二、情态动词。
情态动词本身有一定的字义，如can、may、must、shall、will、ought to、be to、have to、be going to、used to、need、dare等，不能独立充当谓语，必须后跟另一个动词构成
谓语动词。情态动词后面的动词要用原形，第三人称单数时不加-s，如：
He can speak English.
Can he speak English?
He can't speak English.
A unit can measure how hot the pepper is.