Learn German: Du wirst den Trüffel schnell hacken! - You will chop the truffle quickly!

Using werden plus the infinitive creates the Imperative Future. This makes the sentence an absolute, indisputable command rather than a simple instruction. It carries the tone of a strict boss or parent saying, 'You will do this, and I won't hear otherwise!'

How do we know this is a command and not just a prediction about the future?

Context and tone of voice do a lot of the heavy lifting. The exclamation mark is a big clue in writing. Furthermore, a forceful delivery of Du wirst makes the command obvious, and adding adverbs like schnell or sofort (immediately) emphasizes the urgency of the order.

Is den Trüffel singular or plural in this sentence?

It is singular. The word for truffle is masculine (der Trüffel), so the article changes to den in the accusative case because it is the direct object. The plural form is identical (Trüffel), but if it were plural here, the article would be die.

Could we use schneiden instead of hacken here?

You could, but it changes the cooking instruction. schneiden simply means to cut or slice. hacken specifically means to chop or mince into small pieces, which is typically what you do with strong-tasting ingredients like truffles or garlic.