SO is it "ITS" or is it "IT's," you ask?
This is undoubtedly the most common mistake I catch on sites, and I know it can be confusing. But that's because people often think of it in terms of possession when they should be focusing on contraction.
The word "contract" means "squeeze," so that's how we refer to words that we squeeze together by omitting letters, like "don't" (do not) or "isn't" (is not). And that also goes for "it's" or "it is."
Instead of worrying about possession, then, you just need to decide if the phrase makes sense with either "it's" or "it is."
TEST 1 using "it's":
It makes sense when "it is" spelled out like this. OR It makes sense when "it's" spelled out like this. THIS IS CORRECT.
Try with "its" instead: It makes sense when "its" spelled out like this. WRONG. It actually doesn't make sense!
TEST 2 using "its":
It helps when "it is" meaning is clear. OR It helps when "it's" meaning is clear. WRONG. This sentence's meaning is NOT clear!
Try with "its" instead: It helps when its meaning is clear. THIS IS CORRECT.
Oh, and one more thing: Don't get me started on its' because it is NEVER CORRECT. EVER.
All clear? Now go forth and write good copy! It's imperative! :-)
Do you have a grammar question or see a common grammar error you'd like me to address? Contact me at firstname.lastname@example.org, and I'll gladly write a #GrammarMonday post about it.