"The"-the definite article
In Arabic the word ال "el/èl" does what the English word "the" does. It is pronounced [ al ] but the "a" at the beginning is often elided (not pronounced, because the previous word ended in a vowel).
rajuluń = a man
èl rajul= the man
الرجل èl rajul is pronounced ar-ra-jul instead of al-rajul, because "r" is a sun letter. Whenever el is prefixed to a sun letter the transliteration system will indicate this by writing èl instead of el. There is no need to memorize sun letters. e.g. الشمس èl shams is pronounced ash-shams (meaning: the sun).
Add ال el to the beginning of a word, is like adding "the" before a word in English.
Here is an example of what not to do:
Incorrect: ال رجل
Notice that the ل (laam) must connect to the next letter, which was a ر (raa') in the case above.
ال el also has a second use. When you find ال it usually has the same function as "the" in English. It is also often used to talk about a thing in general.
The previous example shows one word meaning "a man." The second is the same word with the prefix "ال" (al), and it means man in general (i.e. mankind). So in English where one says "owls go to sleep at night" in Arabic we would say "the owl goes to sleep at night." This would indicate owls in general and not a specific owl.
So what word holds the same functions as the word "a", you might ask? In Arabic there is no word that holds that function.
So "a man" is just one word, رجل There is no word, or prefix, that has the same funciton as "a".