It seems John is not right in his head.

[tufguy]

We have to use plural form of noun when we are talking about multiple people for example.

1) It seems John is not right in his head.
2) It seems theee people are not right in their heads.
3) John is doing great in his life.
4) My classmates are doing great intheir lives.

We have to use plural every time when talking about more than one person be it anything. Am I correct?

 

[PeterCW]

I will leave the grammar to others once you have corrected the typos.

"Right in the head", not "his head", is a stock expression for somebody with mental health problems. It is used for one or multiple people but is generally considered pejorative and would only be used in casual conversation.

 

[Skrej]

I will leave the grammar to others once you have corrected the typos.

Indeed. I recommend others not to bother investing their time in responding until Tufguy invests some of his own time correcting careless, obvious typos, as he is prone to do.

Tufguy, please demonstrate that you respect and value members' input by checking for basic typos before you post. I've lost track of the number of times you've done this sort of thing, but I'm certain it's in the hundreds; yet you still continue to do it.

 

[tufguy]

Indeed. I recommend others not to bother investing their time in responding until Tufguy invests some of his own time correcting careless, obvious typos, as he is prone to do.

Tufguy, please demonstrate that you respect and value members' input by checking for basic typos before you post. I've lost track of the number of times you've done this sort of thing, but I'm certain it's in the hundreds; yet you still continue to do it.

We have to use plural form of noun when we are talking about multiple people for example.

1) It seems John is not right in his head.
2) It seems three people are not right in their heads.
3) John is doing great in his life.
4) My classmates are doing great in their lives.

We have to use plural every time when talking about more than one person be it anything. Am I correct?

I apologise .

 

[jutfrank]

As you were told in post #2, the fixed phrase is 'not right in the head' regardless of how many people you're talking about:

He is not right in the head.
They are not right in the head.

 

[Tarheel]

We have to use plural form of noun when we are talking about multiple people for example.

1) It seems THAT John is not right in his head.
2) It seems THAT THOSE people are not right in their heads.
3) John is doing great in his life.
4) My classmates are doing great in their lives.

We have to use THE plural every time when talking about more than one person be it anything. Am I correct?

I apologise .

I would say "no matter what" instead of "be it anything". Also, as has already been pointed out, an exception is made for set phrases, like the one in question.