Learn French: Il faut éviter les idées mort-nées.

In compound adjectives, if the first word acts as an adverb modifying the second, only the second word changes to agree with the noun. Here, mort acts as an adverb describing how the ideas were born, so it remains invariable. Only nées takes the feminine plural -es to agree with les idées.

How can I tell when the first word in a compound adjective is acting as an adverb?

An adverb-like first word describes the manner or degree of the second adjective, rather than being an independent trait of the noun. These ideas aren't separately 'dead' and 'born'; they were 'born dead'. Another common example is nouveau-né (newly born), where nouveau also stays invariable.

If a compound adjective is made of two regular adjectives, do both parts agree?

Yes! When a compound adjective consists of two adjectives that both independently describe the noun, both parts must agree. For example, if you were talking about deaf-mute women, it would be les femmes sourdes-muettes because they are both deaf and mute.

Why is il faut translated as 'we must' here instead of 'it is necessary'?

While il faut literally means 'it is necessary', it is the most common way to express a general obligation in French. Translating it as 'we must' or 'one must' usually sounds much more natural in English than keeping the literal phrasing.