Learn Italian: Queste montagne sono belle ma pericolose. - These mountains are beautiful but dangerous.

They are describing montagne (mountains), which is a feminine plural noun. These adjectives belong to the standard group that ends in -o for masculine singular (bello, pericoloso). To make them feminine plural, the ending changes to -e.

How can I tell that montagne is feminine?

The singular form of the word is montagna. Since it ends in -a, it is a regular feminine noun. When it becomes plural, the -a changes to -e (montagne), and all the adjectives describing it must match that feminine plural form.

Does queste follow the same agreement rules as belle and pericolose?

Exactly! The word for this/these (questo) acts just like a regular adjective. It changes to queste here to perfectly match the feminine plural noun montagne.

Do adjectives still have to match the noun when they come after the verb?