Everything was delicious. All presents were good. why was and were

Question

Answer

The indefinite pronoun "everything" is always singular.  Indefinite pronouns beginning with "every-", "any-", "some-" and "no-" are always singular:

everyone, everybody, everything
anyone, anybody, anything
someone, somebody, something
no one, nobody, nothing

With "everything", the meaning is literally "every (individual) thing".  The salad was delicious.  The soup was delicious.  The steak was delicious.  The asparagus was delicious.  The dessert was delicious.  Therefore, each thing (or "every thing") was delicious -- one by one.

Answer

'everything': ONE situation, singular: was
'all presents': SEVERAL presents, plural: were

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