When do we use the -s form of verb?

You use the "-s" form of a verb in the third person singular in the present simple tense. This means you use it when the subject of the sentence is:

* He, she, it, or a singular noun.

Here's a breakdown:

Subject | Verb Form | Example

------- | -------- | --------

He | walks | He walks to school every day.

She | sings | She sings beautifully.

It | rains | It rains heavily today.

The dog | barks | The dog barks at the mailman.

Here are some key points to remember:

* You don't use the "-s" form for the pronouns I, you, we, or they.

* You don't use the "-s" form in other tenses like past simple or future simple.

Examples:

* Present Simple: She plays tennis every weekend.

* Past Simple: She played tennis yesterday.

* Future Simple: She will play tennis tomorrow.

Exceptions:

* The verb to be is irregular in the third person singular. Instead of "be-s," it becomes is.

* The verb to have is also irregular, becoming has in the third person singular.

I hope this explanation is helpful! Let me know if you have any other questions.