Confused between usage of den vs einen?

Question

I am a newbie trying to learn Deutsch. `den` and `einen` are akkusative forms of masculine words. My confusion is when to use `den` and when to use `einen`. e.g. > Ich kaufe den Spiegel vs > Ich kaufe einen Spiegel. This seems to be very simple stuff which I am not able to understand.

Answer 1

Ich habe einen Tisch Zuhause.

I have a table at home.

(Hier :masculine accusative ,we don't know it. It could be any table)

Ich habe den Tisch Zuhause, den du mir gegeben hast.

I have the table you gave me at home.

(Hier: masculine, accusative, we know it)

Answer 2

ein (masculine accusative: einen) means 'a/an' and der (masculine accusative: den) mostly means 'the'.

Ich kaufe den Wagen. – I'm buying the car.

Ich kaufe einen Wagen. – I'm buying a car.