Apple/apples and man/men are the same words - is there any rule for knowing this, or is it just like English sheep and fish, just something to learn and remember?
It's just like the English sheep and fish. You can look at the verb that is conjugated with the noun to make out whether it is singular or plural. Singular सेब would go with है while the plural सेब would go with a हैं.
Someone in another comment said masculine nouns that don't end in ā are the same in the singular and plural!
Can someone explain me the difference between ये and वे
Both of these can be used as 'they'. ये is used when the people in question are nearby (literally or figuratively) and वे when they are far.
Why we can't use Va?
Do you mean 've'(वे)? That would be 'those'.
May I know what is a typo??
Yes why cant