"Aluminum is a metal."
Translation:Το αλουμίνιο είναι ένα μέταλλο.
Such generic sentences in English are actually the exact opposite in Greek and vice versa. The Greek here actually sounds more natural without the indefinite article (ένα) -an alternate accepted answer- and also omitting the definite article (το) would be wrong. There are lots of sentences like this one throughout the course and if you start noticing the pattern, you'll get used to it very soon.