英语语法中，动词的第三人称单数形式有特定的变化规则。一般现在时动词加“s”，一般过去时动词加“ed”，现在进行时和过去进行时动词加“ing”，现在完成时和过去完成时动词加“s”或“es”。但有些动词，如“be”，“have”，“do”，变化规则不同。掌握这些规则对于学习英语语法和口语非常重要。
| 类型 | 规则 | 例子 | | ------------- | ------------- | ------------- | | 一般现在时 | 动词加“s” | He walks to work every day. | | 一般过去时 | 动词加“ed” | She played tennis yesterday. | | 现在进行时 | 动词加“ing” | They are watching a movie right now. | | 过去进行时 | 动词加“ing”和“was/were” | She was studying when he called. | | 现在完成时 | 动词加“s”或“es” | He has two cars. She goes to the gym. | | 过去完成时 | 动词加“ed”或“d” | They had finished their homework by 10pm. |
需要注意的是，有些动词的单三变化规则与上述不同。例如：
| 动词 | 单三变化规则 | 例子 | | ------------- | ------------- | ------------- | | be | is/am/are | She is a teacher. We are students. | | have | has/have | He has a dog. They have two cats. | | do | does/do | He does his homework. They do their best. |
