Any ninja worth his itching powder knows how to use every weapon in his arsenal, and how to turn any innocuous item into a weapon.
The same goes for constructing fiction. SPaG (spelling, punctuation and grammar) is a handy set of three pouches on the tool belt of every modern fiction ninja. Know your SPaG, love your SPaG. Your stories will be choppy and amateurish without at least a basic knowledge of these three tools, and might be so bad that no editor will want to wade through the fetid swamp of your mistakes.
Often what I see in inexperienced writing (not young writing per se: one can be inexperienced at writing at every age) is a lack of grasping the rules behind SPaG. This can lead to inconsistent mistakes, where sometimes the writing is done properly and sometimes the same set of mistakes crop up.
It makes me want to smack everyone. English literacy should not be an option, people! There are rules. Just like for soccer, chess, driving a car, any complex game or task. Either they weren't taught correctly, or they weren't learned correctly. Or both. But there should be smackage for not getting it right. We text, speak and type in this language. How pathetic that we can't get it right, especially if it's the only one we know!
And no, lolcats, 1337speak and txspk are not true languages. They're sub-quality translations for people with a short attention span and/or a desire to avoid carpal tunnel.
Back to SPaG: If you learn the rules that govern writing proper sentences, then you can free your mind from guessing how to put things (excluding artistic expression, of course!) and let it roam over to the imagination factory instead. Or, alternately, you can use some brain cells to care about how your story appears to others who also knows the rules of SPaG.
The most common mistakes that I see involve dialogue. Where the heck do you put that comma: inside or outside? It goes inside, if it's part of what the person is saying.
"I put my comma here," Jasmine said.
The exception is if you're talking about something that you put in quotes, which isn't really dialogue. I could refer to "lolcats", with the comma outside, just like that. Since "lolcats" doesn't have a comma after the word every time you see it, it shouldn't have a comma when you refer to it in quotes.
Another dialogue punctuation issue is whether to have ending punctuation at all. You always do. Unless it's a lolcats issue as above. If they ask a question, you need that [?] inside the quote marks. If your character is exclaiming, the [!] goes inside the quote marks.
If your character finishes a sentence at the same time you do, the [.] goes inside the quote marks, thus:
"I just figured out where to put the punctuation."
If the character finishes a sentence that would normally get a [.] at the end, but you're not done writing your own sentence, it looks like this:
"I just figured out where to put the punctuation," Jasmine said.
If it's a question or an exclamation, then those symbols still go inside the quote mark at the end of the dialogue.
Which reminds me of something else I see too much: capitalization of the pronoun in that dialogue tag. Like this:
"What do you mean, I shouldn't capitalize the 'she' in this dialogue tag?" She asked.
The "She asked" there isn't its own sentence, so it doesn't get capitalized at the start. It's part of the dialogue sentence.
"Oh, so since it's just the same sentence, I don't need to capitalize my pronoun," she realized.
Yes, there we go. There is hope for you yet, my very young paduwan learner.
Be well, all, and Merry Christmas.