It was the cold winter of 2001. 和 It was a cold winter of 2001. 的差別在哪裡？如果不好說明，請提供一些例句。

A/an and the are both articles.
Articles are used to define a noun as either specific or unspecific. To workout, the correct article for the noun, follow the 3 basic rules.
Rule # 1: Every time a noun is mentioned, the writer is referring to:

1. All of them everywhere,
2. One of many, or
3. This one exactly

Rule # 2: Every kind of reference has a choice of articles:

All of them everywhere…(Ø, a/an, the)
One of many……………..(Ø, a/an)
This one exactly…………(Ø, the)

Rule # 3: The choice of article depends upon the noun and the context. This will be explained more fully below.

Definite article(The)
Indefinitino article(A/An)

So in this case, since you know it was in the winter of 2001, you can say the cold winter of 2001, unless there are more than 1 winters in 2001 the article to choose would be "the"