With a third-person singular subject, the verb takes an –s in the present tense. That is, when the subject is 1) only a single entity that could be replaced by “she,” “he” or “it” and 2) the action takes place in the present, one adds the letter s to the verb.
This new computer runs faster than the old one. It saves time.
Compare to the following sentence with plural subjects, in which the present tense does not take the s ending.
These new computers run faster than the old ones. They save time.
If the main verb is separated from the subject by many words, it can be tricky to pick out which word is the subject. In the following, the subject is “Problems” (plural) and the main verb is “fall” (to agree with the plural subject).
Problems for which there exists a solution algorithm that completes in a time that grows in a polynomial fashion with the problem size fall into the set P.