These are some tricky ones, and even people who kinda-sorta know them will still call me up to ask which spelling to use in a particular sentence.  (Which, in fact, someone did just the other day.)

The way most people remember these is that “it’s” with the apostrophe is a contraction: “it is.”  It’s the same with “who’s”: “who is.”  But I remember them from the opposite direction, and if the spellings of these two pairs give you trouble, you might try thinking of them as I do.  To wit:

“It” and “who” are pronouns.  Just like “I,” “he,” “they,” “we,” and “she.”  And the possessive form of a pronoun does NOT have an apostrophe:

• I → my
• He → his
• They → their
• We → our
• She → her
• It → its
• Who → whose

If you can remember that “it’s” and “who’s” are contractions AND that “its” and “whose” are possessive pronouns, you’ll double your chances of getting those spellings right.

You’re welcome 🙂