its vs. it's
"its" = possessive (the dog chased its tail). "it's" = contraction for "it is" or "it has".
your vs. you're
"your" = possessive (your book). "you're" = contraction for "you are".
their/there/they're
"their" = possessive. "there" = location. "they're" = contraction for "they are".
then vs. than
"then" = time sequence. "than" = comparison (better than, more than).