ال makes a noun definite.
However, it is used more widely than English the. Abstract and generic concepts (literature, etc) are made definite, for example with أنا أدرس الأدب (I study the literature). Also, some place names are always referred to with ال, for example المصر (Egypt). Also, ال is used with adjectives depending on what you are trying to communicate. See below for the ways to use ال when phrasing sentences. Memorize the patterns.
There is no "a" or "an" in Arabic. If a word is indefinite, then the "a" or "an" is implied.
|ألباب كبير||the door is big||Complete sentence.|
|ألباب ألكبير||the big door||Incomplete sentence.|
|باب كلبير||big door||Incomplete sentence.|
Note that ال is not the only way to make a noun definite.
By attributing possession (for example, adding ي to the end to connote mine) a noun also becomes definite.
Sun and moon letters
The ل is not always pronounced.