Learn German: Der Regen ist kalt.

es regnet uses the verb regnen (to rain) to describe the action. In this sentence, we are talking about the rain as a physical thing, so we use the noun der Regen (the rain). Just like in English, you can talk about what it is doing, or you can talk about the rain itself.

Why is it der Regen?

Regen is a masculine noun, so it uses der for the word the in the subject position. A helpful tip for learning vocabulary: many weather-related nouns in German are masculine, such as der Schnee (the snow) and der Wind (the wind).

Why doesn't the adjective kalt have an ending here?

When an adjective comes after the verb sein (to be) to describe the subject, it keeps its basic dictionary form. Adjectives only get extra endings when they are placed directly in front of a noun, like in der kalte Regen (the cold rain).

Why is the word Regen capitalized?

In German, every single noun is capitalized, regardless of where it is in the sentence. Because Regen (rain) is a noun, it must start with a capital letter.