hard and fast rule

a hard and fast rule (a strict rule; one which cannot be altered, a rule without exception) — жесткое правило; четкое правило; железное правило; непререкаемое правило; непреложное правило

Example 1: To establish discipline, you have to introduce a few hard and fast rules. — Чтобы установить дисциплину, необходимы несколько железных правил.
Example 2: There are very few hard and fast rules about English grammar.