Learn Turkish: İki tane kahve içtim ama zaten çok yorgundum. - I drank two coffees, but I was already very tired anyway.

It acts as a discourse marker meaning 'anyway' or 'already.' The speaker is explaining why the coffee didn't wake them up. By saying zaten çok yorgundum, they emphasize that their exhaustion was an undeniable, pre-existing state that the coffee simply couldn't overcome.

Could I use önceden instead of zaten to mean 'already' here?

Not quite. While önceden means 'previously' in a strict time sense, zaten is used here to reinforce an argument or justify a situation. It tells the listener, 'Regardless of the coffee, the inherent fact remains that I was very tired.'

Why say iki tane kahve instead of just iki kahve?

Both are grammatically correct and mean the same thing. However, using the counter word tane (literally 'item' or 'piece') is very common in spoken Turkish and slightly emphasizes the physical quantity, as if stressing 'I drank two whole coffees.'

How does the single word yorgundum translate to the full phrase 'I was tired'?

In Turkish, you do not need a separate verb for 'to be' in this context. You simply attach the past tense suffix -du and the first-person ending -m directly to the adjective yorgun (tired) to create yorgundum (I was tired).