You have probably been trying to understand them by using, and in, short isolated sentences eg
A dog is in my yard
The dog is in my yard.
In writing these sentences, I would be choosing which article to use to convey a particular meaning, and the reader would understand this meaning because of the article I had used. But it is difficult for a learner to fully understand this in isolated sentences.
Trying looking at this in a whole passage:
I've been told that dogs are friendly animals. I'm not so sure. They say a dog is man's best friend. I'm doubtful. I have a friend who has a dog and it's not very friendly to me. Whenever I go round to my friend's house, the dog runs out of the house and growls and barks. Dogs are 'friendly' and I'm a man - so why isn't the dog acting more like my best friend when I call?
I have used 'dog' several times, sometimes with an article, and sometimes without. To help me and other respondents to your thread understand your difficulties, why don't you tell us which one's you don't understand. Why did I not use an article at times? Why did I sometimes use 'a' and sometimes 'the'?