articles - Difference between "a" and "the"?

I have a question regarding the usage of articles.
What is the difference between "a" and "the"?

Why do I need to use "a" in "that was a winter I"ll never forget."? Can I use "the" instead?

Also, why do I need to use "the" in "that was the winter we went to Norway"?

Thank you very much!!^^

Answer

The is used to refer to specific or particular nouns; a is used to modify non-specific or non-particular nouns.

the = definite article

a = indefinite article

For example, if I say, "Let's read the book," I mean a specific book. If I say, "Let's read a book," I mean any book rather than a specific book.

Another example, "I went to a forest. *The forest was big.". In this first forest was indefinite one, and after I said the first sentence it became definite one and used the instead.

And in your example, winter is indefinite on "that was a winter I"ll never forget." and become definite and used the in "that was the winter we went to Norway".

Reference