Learn Farsi: همه چیز را در چمدان نچپانید! - Do not cram everything into the suitcase!

The infinitive is چپاندن (chapândan), meaning to cram, to stuff, or to shove. It carries a colloquial, forceful tone, implying that you are forcing items into a space that is too small, exactly like cramming in English. This negative imperative uses the negative prefix نـ (na-) + the present stem چپان (chapân) + the plural/formal ending ـید (-id).

Why does همه چیز (everything) take the direct object marker را here?

Even though everything might sound general in English, in Farsi همه چیز (hame chiz) is treated as a definite, specific entity when it is the direct target of your action. Since you are talking about cramming the entirety of your belongings, it requires the definite object marker را (râ).

Could I use توی (tuy-e) instead of در (dar) for into the suitcase?

Absolutely. در (dar) is standard and works perfectly in both writing and speech, but in everyday spoken Farsi, توی چمدان (tuy-e chamedân) is incredibly common. At this level, you can freely swap them depending on how conversational you want your sentence to sound.

How could I apply this lesson's focus on maximum capacity to this verb?

If you wanted to give the opposite advice and tell someone to pack as much as they possibly can, you would drop the negative prefix and add our lesson's phrase: تا حد امکان در چمدان بچپانید (tâ hadd-e emkân dar chamedân bechapânid), which means Cram as much as possible into the suitcase!