Question

When can I use a or the I didn't know what to use

A

The

Answer 1

A would be used in front of a singular noun that does NOT begin with a vowel. (It indicates one of something). 

I see a spider on your shoulder.(I see one spider on your shoulder).

An would be used in front of a singular noun that begins with a vowel.  (It also indicates one of something). 

I see an elephant at the zoo.(I see one elephant at the zoo).

The plural of both a and an is some.

I see a spider on your shoulder.-----> I see some spiders on your shoulder.

I see an elephant at the zoo.-----> I see some elephants at the zoo.

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The can be used in front of a singular or a plural noun   It has several usages.

a) The most common would be to indicate a noun or nouns that were previously mentioned.

 The spider on your shoulder is red. ----->  The spiders on your shoulder are red.

b) Another common usage would be to distinguish one noun or several nouns from a group by using an adjective.

I see the old elephant at the zoo everyday.-----> I see the old elephants at the zoo everyday.

The has other uses as well, but I am out of time.

Answer 2

we use a/an when we talk about something for the fist time. we use the when we are talking or thinking about one particular thing. Ex.: I sat down on a chair. (one of many chairs in the room). I sat down on the chair nearest the door. (a particular chair).