Student or Learner
Is that necessary here? It is for this reason (that) people will fight.
Either without or without that is fine.
*Not a teacher
I don't think so. My sentence has another effect to express the statement. It emphasizes the sentence and let us know the speaker is more certain when speaking that sentence.
I think maybe it is better to use it in a question, isn't it? , for example: "Can you tell me the reason why it is redundant ?". In this case I think "why" shouldn't be replaced by "that", should it ?
Thank you very much !
The "reason why" is redundant. You will see it and hear it, but careful writers will avoid it.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.
Because you could ask:Can you tell me the reason why it is redundant?
"Can you tell me why it is redundant?"
"Can you tell me the reason it is redundant?"
Using the reason why is common enough but it doesn't add anything, so there is redundancy.