German words ending in -heit

-heit was a noun before it was a suffix. In older German it stood alone and meant something like rank, manner or condition, and it was used both as a masculine and as a feminine. It fused onto adjectives, shrank to an ending, and the feminine is the one that stuck. That history is why this behaves like a piece of grammar rather than a pattern someone noticed. The gender was settled long before any of these words existed.

What it does is mechanical. Adjective in, abstract noun out. frei gives die Freiheit. krank gives die Krankheit. gesund gives die Gesundheit. wahr gives die Wahrheit. sicher gives die Sicherheit. It runs on the odd noun too, so Kind gives die Kindheit.

The useful move is the reverse one. When you meet an unfamiliar -heit word, look for the adjective inside it and the meaning usually falls out: Vergangenheit off vergangen, Gewohnheit off gewohnt, Gelegenheit off gelegen. The ending hands you the article and a running start on the sense at the same time.

-keit is the same suffix in a different coat, chosen by whatever adjective comes before it. Both are die, so you never have to decide which one you are looking at.