Learn German: Den Pullover kaufe ich. - I am buying that sweater.

In German, you can move the thing you want to emphasize to the very beginning of the sentence. Here, the speaker is emphasizing exactly which sweater they are buying. Notice that the verb kaufe stays strictly in the second position, so the subject ich has to move right after it.

Doesn't den usually mean 'the'? Why is it translated as 'that'?

You are right! The basic words for 'the' (der, die, das, and their forms like den) are frequently used to mean 'this' or 'that'. When speaking, you put a strong emphasis on the word Den to show you mean 'that specific sweater' rather than just 'the sweater'.

Why is it Den and not Der?

Pullover is a masculine noun. In this sentence, the sweater is the thing being bought, which makes it the direct object of the verb kaufen. Direct objects use the accusative case, and the masculine accusative form of der is den.

Could I use diesen instead of den here?

Yes, you can! You could say Diesen Pullover kaufe ich. Both mean 'I am buying this/that sweater'. Using diesen is a bit more formal and points very strongly, while using den is extremely common and natural in everyday spoken German.