Now the hard part: with nouns, it depends. There are many rules governing articles before nouns - and just as many exceptions to the rules. The best general rule is that if you're talking about a specific thing, use the, and if you're talking about the general idea of something, use a.
She threw a rock at his head.
She threw the rock she was holding at his head.
In the first, it could be any rock. In the second, it can be only one rock: the one she was holding.
He was lost in a forest.
He was lost in the forest.
In the first, we don't know which forest. In the second, we do - even though it's not explained in this sentence.
But articles are difficult. Just keep listening, and be patient. As you become more familiar with the language, they'll start to make sense.
Student or Learner