In general, "the" is known as the "definite article." When you are talking about a specific, known thing you use "the." She takes the same bus every day.
"A" is the "indefinite article." I will take a taxi. There is not one specific taxi that I am talking about. I will take whichever taxi comes along.
This is general information. Unfortunately the use of articles is more complex and one of the difficult aspects of the language. For instance, we typically describe the activity of banking as "going to the bank" even if we are not speaking of a definite, specific bank. We could land in a strange city and still say we wanted to go to "the bank."
- For Teachers