I have the honor of being trusted enough to edit other people's manuscripts from time to time. Occasionally, I find myself giving mini-lessons in the mechanics of writing as I do so. Here are some examples:
In fiction, dialogue does not go on a separate line just because it's dialogue.
When one character is speaking for several paragraphs, do not put end quotes at the end of each paragraph, save them for the very end of that person's dialogue. Do, however, start each paragraph with opening quotes. This is a subtle indication to the reader that the same character is speaking these paragraphs.
Generally, past tense verbs are preferred over present tense verbs. Whichever tense you choose to write in, stick with that tense. Don't flip between present and past tenses.
If you are writing from Character A's point of view, he cannot tell the reader things he does not know. If B & C secretly meet to discuss something, A must overhear them - or be told about the meeting - in order to tell the reader about it.
'I' is for the subject. 'Me' is for the object of the verb. If you lump yourself with another person, and can't decide if you've chosen the correct pronoun for yourself, try reading the sentence without that other person. 'Mother and me went to the park.' = 'Me went to the park.' I don't know about you, but that grates on my brain. It should be 'I went to the park,' therefore, 'Mother and I went to the park.'
The best source I have found for learning some of these mechanics is the book, Elements of Style by Strunk & White. It is a whopping 85 pages long. It is clear and concise, without a lot of fluff to clutter it up. I try to read it every couple of years, just to remind myself of things.
This book, however, is not written with fiction in mind, so sometimes, a punctuation question comes up that is not addressed in its pages. This past month, I have discovered a blog by 'Grammar Gal', which has answered a couple punctuation questions I had. She seems to know her stuff, and explains it clearly. She has joined my 'resource' list as well.
My dad used to say, "If something is worth doing, it's worth doing right." I have to agree. So if you want to write, brush up on your grammar rules. It makes it much easier on the editor, who can then concentrate on finding any plot holes you might have missed.
Even if you aren't writing books, just emails to friends and family, being aware of the grammar rules will make those emails easier for them to read.