Learn Italian: L'eruzione maggiore ha distrutto non solo la foresta, ma anche il bel lago. - The major eruption destroyed not only the forest, but also the beautiful lake.

Maggiore is the irregular comparative of grande (big), so it usually translates to "bigger" or "greater". However, just like in English, it is often used as a standard adjective to mean "main" or "major" (as opposed to minor or lesser ones). Remember that when talking about siblings or people, it usually means "older".

Why does the sentence use bel lago instead of bello lago?

When placed before a noun, the adjective bello changes its form to match the definite article that the noun would take. Because lago takes the article il (a standard masculine singular noun), bello drops its ending and becomes bel.

Is non solo... ma anche a fixed phrase?

Yes! It translates directly to "not only... but also". Just like in English, it is a very common pairing used to connect and emphasize two related things in a sentence.

What is the infinitive verb for distrutto?

It comes from distruggere (to destroy). Like many verbs ending in -ggere (such as leggere becoming letto), it has an irregular past participle, forming distrutto.