Learn Farsi: شما باید از متهم دفاع کنید.

Because of the word باید (must). In Farsi, modals like باید require the following verb to be in the subjunctive mood. For compound verbs like دفاع کردن (to defend), you drop the می prefix from the light verb کردن to form the subjunctive, leaving you with کنید for the plural 'you' form.

Why do we use the preposition از before متهم?

In English, we 'defend' someone directly, but in Farsi, the verb دفاع کردن usually takes the preposition از (from). You can think of it literally as 'making defense from' the accused person.

I know باید means 'must', but does it ever change to match the subject شما?

No, باید is a fixed modal word in Farsi. It always stays exactly the same, no matter who the subject is. The main verb at the end of the sentence (دفاع کنید) is what takes the personal ending to show that 'you' are the ones doing the action.

How would I change this sentence to say 'You were not defending the accused' using the negative past continuous from our current lesson?

You would remove باید and change the main verb to دفاع نمی‌کردید. To form the negative past continuous of a compound verb, you add the نمی prefix to the past stem of the light verb کردن (which is کرد), and add the plural 'you' ending . The full sentence becomes: .