Learn Russian: Не двигайся, стой тихо!

For не двигайся, Russian strongly prefers the imperfective aspect in negative imperatives to forbid an action entirely. For стой, the imperfective is used because you are commanding someone to maintain a continuous state of standing.

How does the word тихо translate to 'still' in this sentence?

While тихо literally means 'quietly', Russian often uses it metaphorically with verbs of posture or motion to mean 'without moving' or 'calmly'. Telling someone to stand тихо is the standard, natural way to say 'stand still'.

Is двигаться considered one of the special Russian Verbs of Motion?

No. Even though it literally means 'to move', it is not part of the strict Verbs of Motion group (like идти or ходить). It does not have unidirectional or multidirectional pairs, so you just conjugate and use it like a regular Russian verb.

Why do we need the reflexive suffix -ся on двигайся?

The non-reflexive verb двигать is transitive, meaning it is used to move an external object (like sliding a chair across the room). Because the person is moving their own physical body here, the reflexive двигаться must be used.