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]