Learn French: Ils ont expliqué les problèmes principaux.

The singular masculine form of "main" is principal. In French, when a masculine noun or adjective ends in -al, it almost always changes to -aux in the plural. This is why we use principaux to describe the plural noun problèmes.

Wait, problème ends in an "e". Shouldn't the adjective be feminine?

Problème is a classic exception! Even though it ends in an "e", it is actually a masculine noun (un problème). Because it is masculine plural here, the adjective must use the masculine plural form (principaux) instead of the feminine plural (principales).

Why does principaux come after problèmes?

Unlike English, French usually places adjectives after the noun they describe. There is a small group of common adjectives that go before the noun (often describing beauty, age, goodness, or size), but principal doesn't belong to that group, so it follows the standard rule and comes after.

What tense is ont expliqué?

This is the passé composé, the most common past tense in French. It is built using the present tense of avoir (ils ont, meaning "they have") plus the past participle of the verb expliquer (expliqué, meaning "explained"). Together, it translates to "they explained" or "they have explained".