Grammar is probably not the topic you would list as something you wanted to learn more about as you move along your writing journey. Boring? Maybe. It's also something that might make an editor toss your submission in a hurry if you write with too many mechanical errors or grammar no-no's.
The people in my online writers group are not beginners but I have seen many submissions where the writer changes tense in the middle of a paragraph, then reverts back to the original in the next. I will venture to say that most, if not all, of these writers know better but they get wrapped up in the content and errors slip by. It's one very good reason to have others critique your writing for both content and mechanics.
The three main categories of verb tenses are Past, Present and Future. I'm sure you all know that but if you are writing in past tense, then suddenly switch to present in one or two sentences, it throws everything off. Consistency in verb forms is not just a nice thing to do. It's mandatory.
I frequently suggest--even urge--writers to wait a couple days or even longer before editing whatever has been written. It's the little things like verb tense that will show up then. If you start revising and editing immediately after writing the first draft, you're less likely to catch the errors.
Strive for harmony with your subjects and verbs. If you have a singular subject, you need a singular verb. A plural subject requires a plural form of the verb.
I remember grammar exercise sheets that my 7th and 8th grade English teacher handed out at the beginning of every class, five days a week. We had to fill in the blank with the proper form of verbs, adjectives, adverbs and other grammatical subjects. It's a known fact that we learn through repetition. Some students hated those exercise sheets but I know that they helped me learn proper grammar. Ok, I have to confess that I enjoyed doing these exercises while many of my classmates did not. I have always been a word person while many others claim title to being a numbers person. For them, those grammar sheets caused a lot of misery.
There are far too many articles on this topic for me to link to just one. Use your favorite search engine to find them, then take some time to review the proper usage of tenses in writing. You'll find good examples of subject and verb agreement as well as the usage of different tenses of verbs. My aim here today is to make you aware of these two parts of grammar so that you will see them when you write, then edit, your own work.