We use indefinite article 'a/an' when our listener or audience does not know which thing we mean. On the contrary, we use definite article 'the' when it is clear which thing we mean. Let's see the following examples :
We use definite article 'the' when it is clear in the situation which thing or person we mean such as "the light/ the floor/ the ceiling/ the door/ the carpet/ the wall" in a certain room the speaker and listener are sure of. Let's see the following examples :
We also say : the police, the postman, the airport, the fire-brigade, the army, the doctor, the dentist, the bank, the post office.
There is a quite clear difference between a and an. A is used before consonants ( b, c, d, f, g, h, etc. ). An is used before vowels ( a, i, u, e, o).
Note : university is pronounced /yuniversiti/. That's why, it is treated as consonant. So, we do not say or write an university. On the contrary, hour is pronounced /aue:/. It is treated as vowel. So, we do not say or write a hour.
In this exercise you have choose a, an or the to complete the sentences!