There's no other way to say it - there is just no excuse for poor grammar in your captions.
A grammar mistake will instantly make your captions look unprofessional and distract viewers from your message. We HIGHLY recommend you do a thorough proofread of your captions before publishing. And make sure you don't commit these four unfortunately all-too-common grammar sins.
1. There vs Their vs They're
They’re is short for “they are”.
They’re coming over.
Their refers to something owned by a group.
Is that their puppy?
There refers to a place.
We're going there after school.
Make sure you check that you're using the right ones in the right places at the right times.

This is a biggie. The term "a lot" has become very common in writing and speaking. But "alot" is not a word! Make sure there is a space between "a" and "lot". If you’re having trouble with this one, meet the alot monster. It's hysterical and you won't ever make this mistake again.
3. You're vs Your

4. Lose vs Loose

These two words get mixed up all the time despite having totally different meanings. Loose is something that isn't tight. Lose is when you suffer the loss of something. A useful trick is to think that 'Lose' has lost an 'o'.