Grammar seems overwhelming to many people. Don't let it overwhelm you. Just learn a little at a time, and don't worry about the rest. Try to master one rule this year, another next year, etc., and soon you will have eliminated most of the grammar problems from your writing. Mastering a rule doesn't mean learning to recite it. It means learning to find and fix instances of the problem in your writing.
If you are trying to improve your grammar, it may help to have a teacher point out instances of a problem to you. But if you get help, remember to pay attention to only the one problem you are focusing on. If your teacher marks many different problems, focus on just one for a while. The others will wait!
If you try to study all of grammar at once, you'll probably become confused and give up. Here are a few grammar tips to think about. Grammar handbooks have more, but take it slow. Or should we say "slowly"?
Some people put an apostrophe in front of every final s they use, because they are hopelessly confused about apostrophes. This results in more errors than correct forms, because most instances of final s don't need apostrophes. The rule for 's is actually simple, and there is only one exception. Even if you never learn the exception, if you learn the rule, you'll cut down on the number of apostrophe errors you make.
The rule: use 's only for the possessive form. So:
The exception: it's stands for "it is" and is the only 's not used for possession. Honest.
Note: Yes, there are other apostrophes, so the picture is not as simple as it may seem after reading what is written above. But just learn this first and then learn more about apostrophes later. The other rules will not seem as confusing if you are sure about this one. They won't change the facts discussed here. (They have to do with s' not 's. they result in this form of the possessive: parents' means "of parents"; but this is the advanced rule (for making possessives for words that already end in an s and are NOT possessive, like the plural of "parent"), and you can learn this AFTER you already already are comfortable with the simple rule above. And there are also contractions which use apostrophes, but they are not dealing with 's either.)
The rule: when you describe two times in the past, the one further in the past has to be expressed as past perfect.
Example: Yesterday I read the book I had gotten from the library the day before. Had gotten is past perfect and it means I got the book before I read it.
The rule: when you describe something that did not happen in the past but could have happened, you need a special form known as "contrary to fact."
Example: If I had bought a pink hat, I would now be ready for the party. You can tell from this sentence that subject ("I") never bought the hat.