Question

subject-verb agreement
subject - verb agreement means the subject and predicate in a sentence must agree in number.

• subject: who or what the sentence is about
• predicate: what the subject is doing (action)

singular
subject - does not end in \ - s\
verb - ends in \ - s\
plural
subject - ends in \ - s\
verb - does not end in \ - s\
multiple singular subjects
\and\ - plural verb form
\or\ - verb should agree with the nearest subject
exceptions:

• collective nouns: words that act as a singular noun while representing a group
• indefinite pronouns: pronouns that do not explicitly denote a number of subjects
• some singular subjects end in the word s but are not plural. (e.g. \politics\)

which sentence has correct subject - verb agreement?
the apple are in the pie.
the pineapple were in the salad.
the tree is rotten on the inside.

Explanation:

• In "The apple are in the pie", the subject "apple" (singular) should take the verb "is" (ends in -s for singular), not "are" (doesn't end in -s, used for plural).
• In "The pineapple were in the salad", the subject "pineapple" (singular) should take the verb "was" (ends in -s for singular), not "were" (doesn't end in -s, used for plural).
• In "The tree is rotten on the inside", the subject "tree" (singular) takes the verb "is" (ends in -s for singular), which follows the subject - verb agreement rule (singular subject - verb ends in -s).

Answer:

The tree is rotten on the inside.