[it has been cropping up more and more and it's begining to grate.
there refers to a place. somewhere specific (a building) or a general area (country)
"over there", "i went there once", "it's in there"
their indicates possession. it indicates that something belongs to them. not to be used in place of his/her.
"it is their car", "their chair"
they're is a contraction of they are. if you are unsure or too stupid to knock out the 'a' and replace it with ' then leave it as they are.
– if you wrote there, will the sentence still make sense if you replace it with here? if so, you're using it correctly.
– if you chose their, will the sentence still make sense if you replace it with our? if so, you've chosen the correct word.
– if you used they're, will the sentence still make sense if you replace it with they are? if so, you're on the right track!
of refers to a part of something.
off is used to mean away from, it is the opposite to "on". "i got off the bus", "off-road cycling".