Learn Spanish (Spain): Las reglas son muy fáciles.

We use son (from the verb ser) because we are describing a fundamental characteristic of the rules. Being easy is a defining quality of these specific rules, not a temporary state or location, which is when we would use estar.

Why is the adjective fáciles instead of fácilas to match the feminine noun las reglas?

Adjectives that end in a consonant, like fácil, have the same form for both masculine and feminine nouns. To make them plural, you simply add -es to the end, regardless of the noun's gender.

Why does fáciles have an accent mark?

The singular word fácil has an accent mark on the a to show where the spoken stress goes. When we make it plural by adding -es, we keep the accent mark in the exact same place so the spoken emphasis stays on the fa syllable.

There are no sequencing words in this sentence. How does it fit into a lesson about words like primero and después?

When explaining how a game works, you will often start with an encouraging general statement like Las reglas son muy fáciles. Immediately after this sentence, you would typically use your new temporal connectors like primero (first) to explain the setup, and después (then) to explain the next turn.