Its not hard to get them right and they show how careful your being. Even in the 2020’s, computers will not fix these errors for you. Besides, careful editing is it’s own reward. Everything here is wrong, as you have surely noted. Train your eye to focus on potential errors!
Commas and periods are not interchangeable, usually a slight rewording greatly improves clarity like “Avoid run-on sentences because they are very confusing.”
This often makes it harder to follow your idea. This is often the result of stringing ideas together in a logical way as you’re drafting prose, but often you end up not talking about the same thing by the end. This also means it’s easy for the reader to lose track of what “this” really stands for.
That is usually easy to fix by replacing a leading “this” with a more concrete subject: Sentences that start with “this” or “that” make writing hard to follow because they get too abstract really quickly; use a concrete subject instead.
They help your writing flow and save you words.
SMOOTHER: The blue dog, ashamed of its color, repeatedly ran through the car wash.
Note the clunky way is how we think and get ideas into words. The smoother way is what we get from revising and polishing rough written drafts of our ideas.
If you read just the first sentence (or maybe two) of your paragraphs one after the other, you should get a clear picture of the flow of your thinking and narrative. If you have sentences that aren’t clearly and logically moving from one topic to the next, figure out what your paragraph is about, and revise the first sentence to best summarize it. If your paragraph doesn’t clearly elaborate on the main idea you introduce in your first sentence, decide if the paragraph is really necessary. If so, revise it so it makes a clear and distinct point that fits into your narrative.
This was one of my greatest discoveries in writing. Often we write sentences as part of our thinking (remember they are inseparable) that get trapped in our prose. You might keep tinkering with them; you might move them around a bunch trying to find the right place for it. Sometimes you just can save yourself a lot of trouble by just deleting it, or if you’re worried about losing something useful, move it past the end of your essay so that it’s not in your way and you can still see it later if you want. It’s incredibly liberating!
Look for unnecessary is, was, be, or being verbs. See if you can be clearer by using a concrete subject.
Better: The hastily written assignment instructions provided only an unhelpfully ambiguous grading policy.
Active: The king grew impatient.
In the last example, you might deliberately choose the passive construction if your point were that any objective observer would also declare that the class’s very existence should be characterized as boring. If you want to emphasize the agency of the class to bore students, particularly you, but it’s at least conceivable that other students might not find it boring, the active construction would be more precise.
Another way of thinking about it is that the passive voice describes a state of being (how the class is), which is pretty generic and vague. But sometimes that’s what you want. The active voice emphasizes a subject and action (what the class does).
All I’m saying is that you should choose deliberately.