"a" would be used to speak about any random subject. "I saw 'a' deer in the woods" "the" is used to speak about a specific subject. "I saw 'the' deer in the woods, the same one we saw yesterday."

Tom: Do you want A donut? Mona: Why, yes! Thank you. (She starts to eat it and then throws it away in the wastebasket.) Tom: Why did you throw THE donut away? Mona: It was too stale! ***** As you can imagine, sometimes there are exceptions to the rule: Manager: Why are you late to work? Mona: My car broke down. I had to take a taxi to work today. Manager: And? Mona: And THE driver of the taxi got confused and took me to the wrong address. We use "the" because we know that there is only one driver in a taxicab. So Mona is talking about that one PARTICULAR person. ***** This is a true story: One day I was walking down the street in my American city. I passed a store. The store was owned by people who did not speak English well. There was a sign next to the door. The sign said, "If you want to enter this store, ring A bell." Of course, the sign should have said "Ring THE bell." Why? Because the store wanted us to ring THE bell next to the door. A particular bell. ("Ring A bell" means to ring any bell in the world!)