A : Subject Plural / THEY / WE / I / YOU = NO need for "s". Ex : They swim. We drink. You try. I do. Etc
Or [Subject + Verb]
B : Subject not Plural / HE / SHE / IT = NEED "s" / "es". Ex: He swims. She drink. She tries. He does. It breaks. It does. Etc.
Or [Subject + Verb with "s"]
The Dog = He / She (depend, the dog male or female). With subject he/she, they need formula B, with "s" on the verb. Ex: the dog walks.
What about the dogs? With "s" behind "dog" word, meaning the subject is plural. So, we use formula A, no need for "s". Ex: the dogs walk.