Question

the monkeys ate cherries and mangoes plural to singular

Answer 1

Changing from plural to singular structure is easy, but we must also pay close attention to the the form of verb that is being put into the sentence. However, in this question, there no need to change the verb's position or form.

Moreover, the correct sentence in singular form is mentioned beneath.

"The monkey ate a cherry and a mango."

Answer 2

When changing from singular to plural form the words differ

Here are some of the examples  

Monkeys become monkey

Mangoes become mango

Cherries become cherry  

These tend to change like if the vowel is before y then s is added and likewise, there are many other rules followed in singular and plural.