watch的第三人称单数为什么加es

watch主要用作名词、动词，作名词译为“ 手表”，动词为“看，观看”。常与动词keep连用,其后接for, on, over时watch前可加也可不加不定冠词a。例句有：

1、He watched the house for signs of activity.

他注视着那所房子里的动静。

2、Could you watch my bags for me while I buy a paper?