Commas are a tricky little piece of punctuation, and they give many people headaches. In my students’ essays, I frequently see commas where they don’t belong, and I see necessary commas left out. I make it one of my goals to help demystify the comma for my students.
The most common missing comma is the one that comes after an introductory element in a sentence.
You need a comma after an introductory word:
Actually, I’ve never been to Disney World.
After an introductory phrase:
After the storm, many people were without electricity for days.
And after an introductory clause:
Because it was so hot outside, we decided to stay home.
Generally, it’s safe to use a comma in any of these cases. Be careful, though–sometimes what looks like an introductory phrase is actually the subject of the sentence:
Starting an essay without doing your research is never a good idea.
To be or not to be is the question. (apologies to Mr. Shakespeare)
If a sentence starts with a gerund (-ing) or infinitive (to+verb) phrase, using a comma is incorrect!