I'm confused by the grammar of this sentence. Does it translate (roughly) word for word as "fire was necessary for us"?
yes. "il faut" (past "il fallait", future "il faudra") is an idiom where "il" is not "he" . That is an impersonal construction meaning "it is necessary (to have)".
"fallait" = verb "falloir" in imparfait (past tense used for lengthy or repetitive actions and habits).
This verb is "defective" meaning that it is only used with impersonal "il".
Il faut expresses obligation or need: it is necessary / it is needed, primarily.
- I must do it / I have to do it = il faut que je le fasse
- I should do it / I ought to do it = il faudrait que je le fasse
- I had to do it = il fallait que je le fasse
"Il faut" can also be "re-personalized" with the addition of a pronoun in its indirect form: il faut que je fasse = il me faut le faire (I have to do it)
- we needed (some) fire / (some) fire was necessary to us = il nous fallait du feu
Robo-Marie sounds as if she sat on a cactus - "Il nous FAILLAIT du feu."
Why "a" fire rather than "We needed some light." Where is the "a"??
For me, it accepted the more cumbersome, "A light was necessary for us."
I'm wondering about the same thing. 'We needed light' was wrong, yet one of the correct answers was 'We needed fire'.
"du feu" is needed when you are cold or when you are a smoker.
"light" is "de la lumière", needed when it is dark.
I hate to tell you this, but it is rather normal for English speakers to say "we needed a fire" rather than "we needed fire" even if this is outside in the cool night air while we are camping.
I know that in this instance this does not fit the fine rules of French grammar, so it would have to be considered as an idiomatic response.