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

When an adjective comes after the verb to describe the noun (like is mooi), it never gets an extra -e at the end. This is called a predicative adjective. You only have to worry about adding an -e when the adjective comes directly before the noun, as in het mooie weer (the beautiful weather).

Is mooi the standard word to use for "nice" weather?

Yes! While mooi translates to "beautiful" or "pretty", it is the most common way to say "nice weather" in Dutch. You will hear mooi weer all the time.

How do I use words like heel or erg with this sentence?

You can easily make the adjective stronger by putting a degree modifier directly before it. For example, Het weer is heel mooi or Het weer is erg mooi both mean "The weather is very beautiful". If you want to say it's too nice (maybe to stay indoors!), you would say Het weer is te mooi.

Why is it het weer and not de weer?

Weer is a neuter noun, so it takes the article het. There isn't a reliable rule for why "weather" happens to be neuter in Dutch, so het weer is just a combination you have to memorize.