Dreamt about Harry Potter

Harry, Hermione, Ron and I, we four rode on our broomsticks and combated fought with Voldemort.

You don't need the comma after I :)

There was were many magical spells in unknown languages in my dream.

I don’t exactly remember what I dreamt about clearly.

I awoke woke up feeling in a mood of quiet and happy happiness this morning.

When I watched Sherlock Holmes I dreamt that I was a detective and solved a case.

I dreamt about Vampires during those days when I was seeing watching The Vampire Diaries.

But I haven’t seen watched Harry Potter recently.

Try not to begin English sentences with 'and' and 'but' :)

It was is so strange that I dreamt about Harry Potter.

Harry, Hermione, Ron and I all rode on broomsticks and fought with Voldemort.

In my dream, there were many magical spells in an unknown language.

I can’t clearly remember the details of what I dreamt.

People say “what you dream about at night reflects what you think about during the day”.

I dreamt that I was a detective solving cases immediately after watching Sherlock Holmes.

I dreamt about Vampires during the time I was watching The Vampire Diaries.

Harry, Hermione, Ron, and I, we four rode on broomsticks and fought with Voldemort.

In American English, you need a comma after "Ron'. I agree "fought" is better than "combated". I don't think you need "our", and the original "we four rode..." is an acceptable form, but it seems redundant here.

There were many magical spells in unknown languages in my dream.

"Spells" is plural, so I agree with "were". You could use either the singular or the plural with "languages", depending on what you mean.

I don’t exactly remember clearly what I dreamt clearly.

I think that this is more formal written clearly. You could also say:

"I don't remember exactly what I dreamt"

but having both "clearly" and "not exactly" seems redundant.

"About" would be a dangling participle, and I don't think it's needed, anyway.

I awoke in a mood of quietly happy this morning.

This way works, too. I think the other correction is fine.

People say that “what you dream about at night reflects what you think about in day”.

The quote marks are only for exact quotes, whereas this is more like an impression gathered from many opinions.

When I was watching Sherlock Holmes, at night I dreamt that I was a detective and solved a case.

I agree with the first correction, although I would put a comma after the subordinate clause, to make the sentence easier to read. I put "at night" back into the sentence. By putting it before "solved" I made the reference less ambiguous.

I reworded the subordinate clause from the first correction a bit. I think both are okay. Mine just implies that you were watching Sherlock Holmes more than once.

I dreamt about vampires during those days I was seeing The Vampire Diaries.

Only proper names are capitalized, and I agree that vampires should be plural.