A complete sentence has a subject, a verb, and an object. Advertising copy can sometimes survive without one of these elements, but formal prose cannot. Be sure the verb and the subject agrees in number. Remember to keep the tense of your verbs consistent, or your writing might have become unclear. To avoid run-on sentences punctuate sentences properly.
Commas define the cadence of a sentence.
Reading aloud, slowly, can help you identify naturally where they belong. Use the write word, not just the one that looks or sounds rite, to. This is one of the reasons spell-checkers will never replace your brain. Even On tuesdays. Capitalization is one more area where spell-check programs are only marginally useful.