Which sentence is true grammatically? Why?
The first five questions is related ...
The first five questions are related ...
some guys have argued that " The first five questions" is an unit itself and we must use a singular verb. Is it true ?
The noun phrase "the first five questions" has plural number. There is no way around that. Anyone arguing otherwise is simply mistaken.
However, to try to understand how they may have made that mistake, we can explore the situations in which it would become singular. If you use some extra words it can be a unit:
"The set of the first five questions"
"The first set of five questions"
Can be grammatically singular or plural depending on what is said next. Similarly
"The first five-question block"
But in the case of describing them as related to one another, you have to treat it as plural, otherwise the relationship has no parties to be between. If you just say "related" that's the meaning that will be taken. If it's "related to..." then the singular is acceptable, likely more correct, and may be more natural - especially if the last word before the verb is singular.
"The first five questions are related to the industrial revolution."
"The first set of five questions is related to the industrial revolution."
"The first five-question block is related to the industrial revolution."
Some will say:
"The first set of five questions are related to the industrial revolution."
This is wrong by many formal approaches to grammar, but is seen so commonly that correcting it in any environment but the most formal (or a language class) verges on the pedantic. This is because the "set of five questions" is a noun phrase with singular number, but the verb are is in plural form. However, people mix it up thanks to the fact the last word before the verb is plural. It is now so common that it is dubious to claim that it's wrong.
"The first five questions are related to Harry Potter" (or whatever) is the correct usage.
It has nothing to do with 'sets' and stuff, it's just that "they" (the questions) (plural) which therefore takes the 'are' rather than 'is'.
We have 5 questions and they are about Harry Potter.
"The first 5 questions" takes a similar place in a sentence to e.g. "Sam, Jane and George".
For example "Sam, Jane and George are related to Michael".