Learn French: C'est une nouvelle étonnante !

Because it is being used here as a verbal adjective. When a present participle (a verb form normally ending in -ant) acts as an adjective to describe a noun, it must agree in gender and number with that noun. Since une nouvelle is feminine singular, étonnant adds an -e to become étonnante.

How is the base word étonnant formed?

It is formed from the verb étonner (to astonish or to surprise). To create a present participle, you take the nous form of the present tense (étonnons), drop the -ons, and add -ant. This gives you étonnant (astonishing).

Why does the French use une before nouvelle when "news" is uncountable in English?

In English, "news" is an uncountable concept, so we have to say "some news" or "a piece of news." However, in French, une nouvelle is a standard countable noun meaning exactly "a piece of news." To talk about the news in general, you would use the plural les nouvelles.

Why do we use C'est here instead of Elle est, since une nouvelle is feminine?