singular or plural

[beachboy]

My favorite fruit is watermelon/pineapple
My favorite fruit are grapes/apples

What do I have to consider before choosing between singular and plural? The size? What about melons?

 

[Mister Micawber]

.
Interesting. I think the naturalness of both your sentences reflects the fact that in the first we eat segments/pieces of the fruit, while in the second, we eat the whole fruit.
.

 

[shadik1]

I suppose, for grapes we normally wont use as singular(grape).
Usually one says My favourite fruit is apple
My favourite fruits are apple and grapes
or
My favourite fruits are apples and grapes

 

[David L.]

My favourite fruit is watermelon/pineapple/grapes/apples.

"fruit" is singular, and so takes the verb in the singular. Compare:
I like apples and oranges, but tropical fruits are still my favourite.

 

[Mister Micawber]

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We do not usually say, "My favourite fruit is apple".
.

 

[shadik1]

What could be the correct one Micawber?

 

[beachboy]

My favorite fruit is grapes/apples/oranges/watermelon/pineapple
I like grapes/apples, but I don´t like oranges and watermelon.
What about melons/papayas/grapefruit? Shall I use singular or plural? I still don´t know what the rule to choose between singular and plural is... Is there a rule, or it´s only a matter of usage?