What is remarkable is that the principles of subject-verb agreement apply only to finite verbs [the link leaves this page] that are in the present tense and, in a way, to the past form of verbs to be, as it was and was. For this sentence, the subject is “his little sister” and the verb is “to play”. All the above sentences are in the present [external link] and, as you can see, the subject has no influence on the verb. 1. Subjects and verbs must match in number. It is the rule of the cornerstone that constitutes the background of the concept. 6. When two subjects are connected by “and”, they usually need a plural form. The wolves of the pack live in the nearby forest. “Packs” is the theme and is in the plural, and so the verb is “live”, which is paired with. The problem with grammatical rules from the point of view of modern linguistics is that many rules are not absolute.
There are many exceptions to the rules, as we can see here. It can be helpful to bookmark compressed lists of rules like this. . . .