The small word before the thing
Two steps. Start at the first one even if it looks easy — everything later rests on it.
If your first language has no word like a or the, this is not a rule you keep getting wrong. It is a kind of word your language does not have, so it has to be learned rather than translated.
Does the reader know which one?
I saw a dog.The dog was black.
The dog is the same dog in both sentences. The first time, the reader has never met it, so it is a dog. The second time they know exactly which one, so it is the dog. Nothing about the animal changed. What changed is what the reader knows.
English puts a small word in front of most single things. Which word depends on one question, and only one: does the person reading already know which one you mean?
Choose a, the, or nothing.
I saw ___ cat in the garden.
___ cat was sleeping.
My sister is ___ doctor.
Please close ___ door.
I like ___ music.
Can you count it?
You can have one book or three books. You cannot have three advices. Uncountable nouns are measured, never counted, and they never take -s. This decides much or many, and whether a is even allowed.
Sort each word.
bookchairapplefriendadviceinformationfurniturehomeworkThese are the ones to learn by heart. Most languages count them, and English does not:
adviceinformationfurniturehomeworknewsmoneyresearch