You're correct in thinking that the singular form of the verb should be used, but the singular past tense of 'be' can be 'I was', 'You were', or 'He/She/It was'. As the verb follows 'you', it should be 'were'.
His argument is that "bananas" refers to the substance that makes up the fruit, rather than the units, pretty much as chicken refers to the substance, or flesh, in "I like chicken" - not chickens. So he says bananas is equal to chicken in this example.
I want to debunk his argument. He agrees that we'd say, "I like chicken" (not chickens) – but that is the singular form of the word. So I wonder why he thinks we'd switch to the plural, and say the very ungrammatical, "Bananas is sweet fruit." He is dishing out bad guidance here.
I agree with Andrew; these are correct ways to say this:
This one is also valid:
- The banana is a sweet fruit.
(Here, the definite article the indicates we are referring to all bananas, not one particular banana.)
One could even say:
- Bananas are sweet fruits.
- Banana is sweet fruit.
I think both of those are less common, but they are both grammatical.
But the one he is using:
should be avoided in all cases.
Going back to his chicken argument, I can say either one of these:
- Chicken is healthy meat.
- Banana is sweet fruit.
But I would not say, "Chickens is healthy meat," nor would I say, "Bananas is sweet fruit."
Best Answer
Actually, it's not a partitive construction. Partitives normally require a definite noun phrase, the kind with the determiner "the", as in Some of the sugar is missing, or a genitive pronoun, as in Some of his food was eaten. But in Two teaspoons of sugar, "sugar" is not a definite noun, so this is simply a noun phrase with teaspoons as head and of sugar as complement.
Partitive is intended to denote a part rather the whole thing. Some of his food is a partitive construction in that the partitive his food denotes a quantity and Some of his food denotes a subquantity of that quantity. We understand it to mean Some food from his larger quantity of food was eaten.
Measure phrases like the plural two teaspoons in your example can be conceptualised as referring to a single measure which can override the plural form in determining the form of the verb. Which is why singular "is" is fine.