What is the principal difference between “taḥfaẓu al-amānah” and “iḥfaẓ al-amānah”?

Supplementary Practice 4: Audience, Emphasis, Restriction and Rhetorical Judgment

• The first asserts conduct and the second requests an action
• The first is an imperative and the second past tense
• They are two forms of restriction
• They must be the same grammatical form