usually,always后面第三人称单数加不加S?放了一个暑假,什么都忘了．请把第三人称单数要加S不加S的情况说明一

一般现在时,第3人称单数要加如
He always(usually) does his homework after dinner.
He never throws rubbish about.
一般过去时,即使有usually,always等,也不用加s,而是用过去式,如
He always(usually,never) did his work after dinner last year.
如果前面有情态动词,如can,will,would,may,can,could,should,might,must,need等,或是不定式（动词前加to）的,就不用加s
如
He can always do his homework by himself.
He will usually do his homework after dinner.
His teacher asks him to do his homework after dinner.
有时候,一个句子里有两个人物,但动词还要加s,是因为主语只有一个,如
He usually GOES to the park with his classmates.
句子中,classmates不是主语,he是主语,所以要加s(因为go是特例,要加es)
常见的就这些