Agreement* is the word for the correct matching of the subject and the verb in a sentence. (Another way to say this is that the subject and verb go together.) English has very few verb endings and the only one that learners really have to worry about is the -s ending in the present simple tense. As an example let's look at the verb to work:
|1st person||I work||We work|
|2nd person||You work||You work|
|3rd person||He works|
It can be seen that the -s is needed in the 3rd person singular*. It has to be: My father works in a bank. My mother drinks green tea for breakfast every day. Oil floats on water.
Of course, the same rule applies when the 3rd person subject is a pronoun. So you have to say: He works in a bank. She drinks green tea for breakfast every day. It floats on water.
You also need to remember the -s in sentences with relative pronouns. So, for example, it must be: Do you know the man who lives in the next apartment? A carnivore is an animal that eats other animals. And don't forget the -s when the verb is used as an auxiliary: Does your mother like English food? She has forgotten her homework again.
Learners often make agreement mistakes when the noun is unexpectedly singular or plural. So, for example, everybody, news and data are singular, whereas people, police and jeans are plural:
Similar mistakes of agreement occur between such nouns and their pronouns. It has to be:
(* Modal verbs like will, would, may, must etc. do not have an -s in the 3rd person singular.)
* You may find that your grammar book or online article uses the term concord to refer to agreement.