I have come across the phrase 気をつけなければ but I do not understand how it works. Google translate says that it means "I have to be careful" but I can't find which conjugation it is using. To me it looks like the provisional form but I don't understand how that fits here. The full sentence is 両親を起こさないように気をつけなければならなかった.
V-nak-ereba is an abbreviation of V-nak-ereba naranai. The literal analysis of this is "if (-ba) I don't (-nai) [verb], it won't do (naranai)":
- 気をつける be careful.
- 気をつけない not be careful.
- 気をつけなければ… if they're not careful… (it won't do).
- 気をつけなければならない if they're not careful, it won't do (=they have to be careful).
Though that's the literal construction, the full expression is just the common, everyday way of saying "I have to [verb]", which is the idiomatic translation. Native speakers don't usually analyze its parts, just like English speakers don't usually think of breakfast as the interruption of fasting, etc.