1. I ate a piece of banana(s).

2. I ate some pieces of banana(s).
3. I handed two pieces of apple(s) to the boy.
------------------------------
Do we have to use a singular noun or a plural noun in each sentence?

a piece of ONE banana

well, again ONE banana or just the singular type of fruit

again not two apples but two pieces of apple

In order to determine whether a singular or plural noun should be used in each sentence, we need to consider the context.

1. "I ate a piece of banana(s)." - In this sentence, you ate a single piece of banana. This suggests that a singular noun, "banana", should be used. If you are referring to multiple pieces of banana, the sentence could be revised to "I ate some pieces of banana."

2. "I ate some pieces of banana(s)." - This sentence indicates that you ate multiple pieces of banana. Therefore, a plural noun, "bananas", should be used.

3. "I handed two pieces of apple(s) to the boy." - Here, you handed two pieces of apple to the boy. Similar to the first example, this suggests the use of a singular noun, "apple". If you wish to refer to multiple pieces, you could revise the sentence to "I handed him two pieces of apples."

It's important to consider whether you are referring to a singular or plural quantity and adjust the noun accordingly.