在英语中，当主语是第三人称单数（he, she, it）时，动词需要加上-s或-es来表示单数形式。这个规则适用于一般现在时、一般将来时、现在完成时等时态。以下是一些具体的例子和规则：

一般现在时

He works at a company.

She watches TV every night.

It rains in the morning.

一般将来时

He will go to the park.

She is going to study abroad.

It will snow next week.

现在完成时

He has finished his homework.

She has lived here since 2010.

It has rained for three days.

特定词尾变化

动词以-s, -x, -ch, -sh结尾时，加-es：

He catches a ball.

She teaches math.

It washes the car.

动词以-o结尾时，加-es：

He watches a movie.

She plays the piano.

It flies a kite.

动词以辅音字母+y结尾时，将y变为i再加-es：

He studies hard.

She tries new foods.

It carries a bag.

根据这些规则，当主语是第三人称单数时，动词需要加上-s或-es。这个规则是英语中的约定俗成，适用于大多数情况。需要注意的是，有些不规则动词的变形并不遵循这些规则，需要单独记忆（例如：be动词的is, am, are）。