When the subject of a sentence is third-person singular, its verb takes an -s or -es ending in the present tense.
As the chart indicates, the pronouns he, she, and it are third-person singular; so are all singular nouns (such as child) and indefinite pronouns (such as everyone).
In casual speech or informal writing, the -s ending required by standard English is sometimes omitted.
The subjects, Shelby and instructor, are third-person singular, so the verbs must end in -s.
You may be tempted to use the -s ending on verbs with all singular subjects. But the -s ending is used only when the subject is third-person singular (he, she, it, singular nouns, indefinite pronouns).
Likewise, the -s ending is not used on a verb when the subject is plural. It is used only with third-person singular subjects.
Subject-verb agreement at a glance
When to use the -s (or -es) form of a present-tense verb
Present-tense verb endings
Exercises:
Standard English verb forms 1
Standard English verb forms 2
Standard English verb forms 3