Learn Portuguese (Portugal): Nós temos treze canetas na gaveta.

Temos is the "we" form of the irregular verb ter (to have). Since the subject is nós (we), we use temos. Notice that unlike regular verbs, ter changes its spelling quite a bit in other forms (like tenho for "I have"), but temos is the specific form you must memorize for "we".

Can I drop the word Nós from the beginning of the sentence?

Yes! In Portuguese, the verb ending usually tells you who is doing the action. Because temos strictly means "we have", saying Nós is completely optional. You will often hear native speakers simply start the sentence with Temos treze canetas...

Why does "in the" translate to na?

Na is a mandatory combination of the preposition em (in/on/at) and the feminine word for "the", a. Because gaveta (drawer) is a feminine noun, we must combine em + a to make na.

Do I need to change the ending of treze to match the feminine word canetas?

No, the number treze stays exactly the same. In Portuguese, only the numbers one and two (like um and uma, or dois and duas), as well as the hundreds, change to match masculine or feminine words. For almost all other numbers up to ninety-nine, the word does not change based on gender.