Learn Farsi: پسر کوچک هیچ‌چیز در قلعه پنهان نمی‌کرد. - The little boy was hiding nothing in the castle.

To make the Past Continuous negative, you just add the negative prefix ne- in front of the mi- prefix. So, mi-kard (was doing) becomes ne-mi-kard (was not doing).

The translation says "was hiding nothing", but the Farsi verb نمی‌کرد is negative. Is this a double negative?

Yes! In Farsi, whenever you use "none" words like هیچ‌چیز (nothing) or هیچ‌کس (nobody), the verb must be negative. So you literally say that the boy "wasn't hiding nothing."

Where is the "was" part of "was hiding"?

Farsi doesn't use a separate helper verb like "was" or "were" for the Past Continuous tense. The continuous "was doing" meaning is entirely created by the mi- (or ne-mi-) prefix attached to the past tense verb.

How do I connect پسر (boy) and کوچک (little) when speaking?

You link them using the ezafe (an unwritten "e" sound). Because کوچک (little) is an adjective describing پسر (boy), you must pronounce them together as pesar-e kuchek.