Learn Hardcore Russian: Это просто правило.

Это is a demonstrative pronoun in the neuter nominative form. It acts as the “placeholder” subject meaning “this (is)”, introducing the idea you’re about to describe. In English it corresponds to “this is.”

Why is there no verb like быть or English “is” in Это просто правило?

In Russian, the present‑tense copula быть (“to be”) is normally omitted. So Это просто правило literally reads “This simply rule,” but it is understood as “This is just a rule.” In past or future tenses you would use было, будет, etc., but not in the present.

What part of speech is просто, and what exactly does it modify here?

Просто is an adverb meaning “simply,” “just,” or “merely.” It modifies the predicate as a whole, telling you that it’s nothing more than a rule. It’s indeclinable (it never changes form) and does not agree with gender or number.

Why isn’t it простое правило if I want to say “a simple rule”?

Простое (neuter nominative adjective) would agree with правило and mean “simple,” i.e. “простое правило” = “a simple rule.” But in Это просто правило, просто is an adverb (“just”), so the phrase means “it’s just a rule,” not “it’s a simple rule.”