Learn Farsi: ما مدرک جمع‌آوری نمی‌کردیم.

It combines the negative prefix ne-, the continuous prefix mi-, the past stem of کردن (kard), and the "we" ending -im. Notice that before mi-, the negative prefix is always pronounced ne- instead of na-.

Why doesn't the negative prefix attach to the very beginning of the verb, like نجمع‌آوری می‌کردیم?

جمع‌آوری کردن (to collect) is a compound verb made of a noun (جمع‌آوری) and a helper verb (کردن). In Farsi compound verbs, prefixes like ne- and mi- only attach to the helper verb. The first word always stays exactly the same.

The translation says "evidence," but is مدرک singular or plural here?

It is singular. In Farsi, when you talk about a general category or an indefinite concept in a broad sense (like "evidence" or "proof"), you usually just use the bare singular form of the noun without any plural endings.

Could we drop the pronoun ما (we) in this sentence?

Yes, absolutely! Since the verb نمی‌کردیم ends with -im, it already tells us the subject is "we." You could just say مدرک جمع‌آوری نمی‌کردیم and it would mean exactly the same thing.