Punctuation. If you went to school before 1970 you're probably fine with it, but somewhere it got lost in education and it matters. Why?
a) it follows the rhythm of speech so indicates when to pause etc.
b) it makes your writing clear and easy to read.
c) you look as if you're lazy, careless or stupid if there are lots of punctuation mistakes. Sorry, but that's true. And especially so if the reader was educated before 1970, because poor punctuation screams off the page.
d) editors and agents are looking for reasons to discard your work and poor punctuation gives them an excuse.
Sorry, but you just have to learn it! Especially if you're being picked up on it all the time. Get a good book eg Strunk and White. And if you can't be bothered to learn, then find yourself a good line editor, a friend who will delight in fixing your punctuation before you send your work out. They are around.
In student work I notice a lot of mistakes around dialogue.
'Speech and the punctuation pertaining to the speech stays WITHIN the quotes,' she said.
'But the sentence is completed by the "she said" so the full stop comes after that.' He paused, thinking he'd got a capital H because this was starting a new action, that of pausing, after he'd finished his previous action of speaking, which therefore merited the full stop.
'The same is true even if the dialogue is finished with an exclamation mark!' she added. 'Or a ?' she continued, bossily.
Another common error is putting in commas when they should be full stops and vice versa. Think about how you speak when reading aloud. A full stop comes when you take a proper breath, commas are just the briefest of pauses. Or, think about what happens when you read out a list. Try reading this out:
'I went shopping and I bought a cat, a mouse, a bit of cheese, two tomatoes and a potato.'
If you listen you can hear your voice goes up slightly at the end of each word that's separated by the comma, and down when it comes to the end with the full stop. Punctuation is actually quite logical, it's not arbitrary sets of rules sent to try us.
(BTW I could have gone for a ; in that sentence instead of the comma as they are two complete sentences in themselves, linked by a common thought. And if I was speaking, I'd have a slightly longer pause between the two than you'd usually have with a comma pause.) Note full stop within the brackets as it's a complete sentence/thought within itself.
Punctuation shows us how people are speaking, where they are breathing, what the emphasis is, where the rhythm is. That's why it's important. And you don't want to look lazy, careless or stupid either, do you?