Learn Dutch: Het weer is zo mooi. - The weather is so beautiful.

In Dutch, when an adjective comes after the noun it describes (with a verb like is in between), it never gets an extra -e at the end. You would only use mooie if it came directly before a noun, like in het mooie weer (the beautiful weather).

The lesson teaches words like heel and erg. Could I use those here instead of zo?

Absolutely! zo means "so", while heel and erg mean "very". You can swap them easily: Het weer is heel mooi means "The weather is very beautiful." These degree modifiers always go right before the adjective they modify.

Doesn't the word weer also mean "again"?

Yes, it does! weer is a word with two very different meanings. As a noun (het weer), it means "the weather". As an adverb, it means "again". You can easily tell them apart here because it has the article het right in front of it.

Why is it Het weer and not De weer?

weer happens to be a neuter word, which means it always takes het instead of de. There is no strict rule for this, so it is always a good idea to memorize the article (de or het) whenever you learn a new Dutch noun.