the subject and the verb in the English sentence must match
if the subject is plural, then the verb form used is also plural
then if the subject is singular, then the form of the verb used must also be singular.
There are 2 areas where both the subject and the verb can go wrong,
the first on writing, and the second on speaking skills
when the verb is plural, it must be added with an 's' to indicate that the verb is plural. otherwise the verb is singular, there is no added 's'
example:
car becomes cars
babies become babies
but if you use a plural subject, there's no need to add an 's' to the verb.
"the cars look expensive" (the noun shows the plural)
"the car looms expensive" (noun denotes singular)
when the noun is singular, the verb needs to be added with an 's'
if there is an additional verb in the sentence (auxiliary verb), such as do / does in the simple present, am / is / are / was / were in the contunious tense, have / has in the perfect tense. then there must be a match between the subject and the verb.
example:
we are going to the beach
he is going to the beach
there are also modal verbs such as, may, could, will, must, should. and they are also adjunct verbs, but have different subject and verb rules than before. verbs followed by a modal verb, not using the extra 's'