Learn Dutch: De bergen zijn heel mooi. - The mountains are very beautiful.

When an adjective comes after the verb to describe the noun (this is called a predicative adjective), it never gets an extra -e at the end in Dutch. So even though de bergen is plural, we just use the basic, uninflected form mooi.

Does heel always mean "very"? I thought it meant "whole".

Heel can mean "whole" or "entire" when it is directly attached to a noun. But when it is placed right before an adjective (like mooi), it acts as a degree modifier and translates to "very".

Could I use erg instead of heel in this sentence?

Yes! Erg is another degree modifier that means "very". Saying De bergen zijn erg mooi would mean the exact same thing and is just as common.

What is the singular form of bergen?

The singular is berg (mountain). Dutch commonly forms plurals by adding -en to the end of the singular noun. Because it is plural here, it takes the definite plural article de and the plural verb zijn (are).