Learn French: La zone est inhabitable parce qu'elle n'a pas d'atmosphère respirable. - The zone is uninhabitable because it does not have a breathable atmosphere.

Actually, it is a major false friend! In French, the prefix in- makes it negative, so inhabitable means uninhabitable (unlivable). If you want to say a place is livable or able to be inhabited, you just use habitable.

How would we apply this lesson's grammar to say: 'If the zone is uninhabitable and (if) it doesn't have a breathable atmosphere...'?

You would start with si for the first condition, but use que plus the subjunctive to avoid repeating si for the second one: Si la zone est inhabitable et qu'elle n'ait pas d'atmosphère respirable... Remember that the substituting que always forces the subjunctive, even if the original si clause is in the present indicative.

Why is it pas d'atmosphère instead of pas une atmosphère?

This is a quick refresher on negation: in French, the indefinite articles (un, une, des) almost always change to de (or d' before a vowel) after a negative verb. Because the verb avoir is negated here (n'a pas), the expected une becomes d'.

The d' hides the gender of atmosphère. Is it masculine or feminine?