Die wichtigste Regel des Spiels ist einfach.

The most important rule of the game is simple.

Breakdown of Die wichtigste Regel des Spiels ist einfach.

• sein: to be
• einfach: simple
• das Spiel: the game
• des: the; (masculine or neuter, genitive)

Questions & Answers about Die wichtigste Regel des Spiels ist einfach.

Why is wichtigste used here instead of something like mehr wichtig?

In German, you form the superlative of an adjective either attributively by adding -ste plus any needed inflection (wichtig → wichtigste) or periphrastically in predicate position with am + adjective + -sten (e.g. “die Regel, die am wichtigsten ist”). You do not say mehr wichtig—mehr marks the comparative (wichtiger, “more important”).

Why is it die wichtigste Regel and not die wichtigste Regeln?

Regel (rule) is feminine and singular here. If you meant “rules” (plural), you would say Regeln. Since the sentence refers to one rule, it stays Regel.

What case is des Spiels and why is it used?

Des Spiels is the genitive singular of das Spiel. German uses the genitive to indicate possession or close relationship (“the rule of the game”). The article das becomes des, and the noun gets an -s.

Why does Spiels end with -s? And why no apostrophe?