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These are correct:
My favorite sport is soccer.
My favorite food is pizza.
These are also correct: but why?
(What I am looking for is a grammatical rule that explains this)
My favorite animal is dogs.
My favorite fruit is strawberries.
They sound correct but look weird...
_____
These would not be correct:
My favorite animals are dogs.
My favorite animal are dogs.
My favorite fruits are strawberries.
My favorite fruit are strawberries.
You must use singular "is" because "animal" is the subject, and "animal" is singular. The object must also be singular to agree with the subject of "to be". The only exception is for objects which can commonly be thought of together as a singular collection. This is true for "strawberries", but not "dogs", because we think of eating "strawberries" as a whole rather than the individual strawberries.
Favorite 
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