I do a lot of critiques as part of the requirement to belong to my online writing group. There is one punctuation error that I note over and over. It involves the use of a comma after an introductory phrase or clause.
If you use an introductory phrase, place a comma after the phrase and prior to the independent portion of the sentence. Note that I did so in the preceding sentence. Here are a few examples:
1. While Molly picked tomatoes, Sam carried buckets of water for the garden.
2. For instance, I use only the best quality meat when making this recipe.
3. When Jill came to town, Jack remembered the fun they once had running up a hill.
Note that the red part of the sentence can stand alone. It is totally independent from the blue words. Using the comma after the blue section sets it off from the main sentence. It also gives a reader an automatic pause instead of having to breathlessly read the full sentence. Try reading each one aloud with and without the comma. You'll soon see the difference.
Some of us learn the punctuation rules in school and then promptly forget or ignore them once we're grown-up writers. You might write a wonderful story that an editor could reject because it is filled with improper punctuation usage. OK, being realistic, if the story is good enough, the editor might either ask you to polish it and resend or offer some editing help. The latter is less likely in today's writing world.
Another situation could be that the teacher presented the rules day after day in your English grammar class but you never really 'got it.' Not to worry. Not everyone is a grammar guru! If you didn't learn the rule way back in school, you can still master the comma rules today.
There are a whole lot of those punctuation rules and one huge section is the comma. A comma after an intro phrase of clause is not the only one we need to be cognizant of. But it's one that people forget more than any other.
There is a new school of thought on to use or not use the comma when connecting two independent sentences when there is a conjunction word between them--ie and, but, or, nor. The old rules tell us to put the comma prior to the conjunction. The newer method is to eliminate the comma. I have adopted the newer usage but it's been difficult to relearn this rule. I also think that, when reading aloud, it makes it more difficult to read the full sentence. That comma is a natural resting place.
A few examples of this rule showing both ways to write the sentence:
1. John and Mary were longtime friends, but they fought like cats and dogs.
John and Mary were longtime friends but they fought like cats and dogs.
2. I wanted to ride my bike to the store, but the tire looked totally flat.
I wanted to ride my bike to the store but the tire looked totally flat.
3. Susan aced the test Monday morning, and she flunked the one on Tuesday.
Susan aced the test Monday morning and she flunked the one on Tuesday.
Whether you use a comma or not doesn't make a huge difference in these compound sentences. It's a matter of style.
Yes, there are so many piddly things to learn when you want to write. Even so, it's well worth your time and effort so that you acquire the mechanics of good writing.