Avoid sentence fragments.
A fragment is when the sentence is incomplete, usually lacking a verb or a subject, or having the only verb end with -ing (called a progressive verb). To fix it, either add in the missing part, or reattach it to the sentence it should have been attached to.
When you begin a sentence with a conjunction, such as "as," "when," "because," "so," etc., the sentence MUST have a second clause: otherwise it is incomplete.
Because I went to the store. -- not a sentence
Because I went to the store, I was able to buy some food. -- now it's a sentence
Every sentence and independent clause must have at least one non-progressive verb (one that does not end with -ing), or it is not a complete sentence.
Avoid run-on sentences.
A run-on is when two or more complete sentences have been smushed together without a conjunction or proper punctuation. Here are some ways to fix a run-on sentence such as I went for a walk it was hot outside:
Put a conjunction in the middle: I went for a walk while it was hot outside.
Put a conjunction at the beginning and a comma in the middle: While it was hot outside, I went for a walk.
Turn it into two sentences: I went for a walk. It was hot outside.
Rewrite: As I was walking outside, I noticed that it was getting hot.
Blathering is when you string together a bunch of sentences with conjunctions (and, but, or, for, neither, nor, yet, so, then). In general, if you have used more than one conjunction in a sentence, be suspicious — check it over.
Pick a tense and stick to it.
The tense of a verb indicates whether it is happening now (present), happened before now (past), or will happen after now (future).
Avoid starting sentences with conjunctions, such as then, and, so, or, because, except, until, or like.
It often makes an incomplete sentence unless you have a second half (see example below) and, even when it doesn't, it’s not precisely wrong, but it’s one of those things you only do when you have a particular reason for doing it. If you are starting a sentence with a conjunction, it usually means this should be part of the sentence right before it.
Wrong - Until I went to the store.
Right - Until I went to the store, I had no food in the house.
Also right - I had no food in the house until I went to the store.