Question

How to know when we have to write "a" or "the" in between a sentence?

Answer 1

"the"is used to specify a particular noun wheres "a" is used to specify non specific nouns it is mostly used for constants. Example- lets use that thing,it is specifing something so we use the.

Answer 2

"the" is used
1)referring to someone or smth that is already mentioned in a sentence before.
2)before a noun which is known.
3)with superlative adjective which make a noun definite.
4)before adjective used as noun.
5)before nouns which are unique.
"a" is used
1)when the noun has a sense of being one.
2)in referring speed, price or ratio.
3)to make a common noun out of a proper noun.
4)with names of occupation