is this senetence put right? or how do i fix it?
Commas are very important to add to a sentence where they are needed because if you don't properly use them, the reader will be confused.
It's very awkwardly phrased. Try this:
It's very important to put commas in a sentence where they are needed so readers are not confused.
You need a comma after needed.
how about
John went to the store, then called Kristy.
That's a run-on sentence. How do you think you can fix it?
john went to the store, then when he got home, he called Kristy.
Nope.
John went to the store, and then called Kristy.
The sentence you provided is grammatically correct. However, I would recommend a small revision to make it clearer and more concise. Here's the revised version:
"Commas are crucial in sentences because without proper usage, readers may become confused."
In this revised sentence, I removed the phrase "to add" since it is implied that commas are added. I also combined the phrases "very important" and "needed" to create a more concise sentence.