When we use them?

We use there is to say that something exists or is in a place
There is a bridge in the park.

We use there is for singular nouns and there are for plural nouns.

There is a restaurant in the station.
There are two cafés in the shopping centre.

an (before a singular noun beginning with a vowel sound) = an egg, an axe

an

​a (before a singular noun beginning with a consonant sound) = a ruler, a pencil

​a

Articles:
the (before a singular or plural noun) = the book, the bags

SINGULAR AND PLURAL NOUN

​ a singular noun refers to only one person or thing
for example: There is a man in the room

a plural noun refers to more than one person or thing
for example: There are 3 men in the room

Preposition

A preposition is a word or group of words used to link nouns, pronouns and phrases to other words in a sentence. Some examples of prepositions are single words like In, On, At, Under, Below, Above, Next to, Behind, In front of .