can you please tell me guys where i mistake in this sentence?

[...a]

"It's hurt so much pain when someone already known your past but still,they always reminds you that you're nothing without them."

 

[JMurray]

not a teacher

"It's hurt so much pain when someone already known your past but still,they always reminds you that you're nothing without them."​

There are several mistakes. I'm not sure I understand exactly what you want to say, but perhaps this will help.

"It hurts so much when somebody already knows your past, but they still keep reminding you that you're nothing without them."

 

[cereal_chick]

A more comprehensive critique:

"It's hurt so much pain when someone already known your past but still,they always reminds you that you're nothing without them."

"It's hurt so much pain" = "It's hurt" is to me "it has hurt", which does not make sense with the rest of the sentence. Secondly, one cannot hurt pain; although I understand what you are trying to say, it does not work: it would be better said as, "it hurts so much".

"when someone already known your past" = This construction (with "known") requires the verb to have, in this case conjugated to has, to make "when someone has already known..." – again, this does not work, you must say "when someone already knows your past..."

"but still,they" = A comma is attached immediately to the last word in the clause (what the comma denotes) but there must be a space before the next word! Correct: "but still, they..."

"they always reminds" = The correct form of the verb is "they remind", because "they" is plural. It may help you to think of English present conjugation like this: the "s" (or the plural) must go on either the subject (the doer of the action) or the verb (the action word); but not both. A correction for this could be "they always remind..." or as has been suggested above (which I personally think works better stylistically) "they still keep reminding..."

Of the title of the thread: Capital letter at the start of the sentence, capital I ALWAYS when it occurs by itself, and finally, "mistake" is not a verb.
"Can you please tell me guys where I made a mistake in this sentence?"

[Not a teacher]

 

[emsr2d2]

... and finally, "mistake" is not a verb.
"Can you please tell me guys where I made a mistake in this sentence?"

[Not a teacher]

Whilst it's true that, in this context, "mistake" is not a verb, the verb "to mistake" does exist within "to mistake someone/something for someone/something else".