We use following determines in front of:
singular countable nouns:
a/an
the
another
the other
uncountable nouns:
the
some
any
no
a lot of
much
all (of the)
plural countable nouns:
the
some
any
no
many
several
a lot of
all (of the)
(the) other
We use a/an when:
we mention something for the first time:
There is a new accounter in our company.
We saw a new teacher today.
it does not matter which particular person/thing we are talking about, e.g. when we mention this person/thing as an example of a group or category:
Did you buy a pineapple?
Can I have an orange?
We use the when the person we are talking to knows precisely which person/thing we are talking about and can easily identify them/it: