"We have to build a higher wall."
Translation:Il faut construire un mur plus haut.
"Nous devons" is accepted, but "il faut", which is similar to "it is necessary (to)" is an impersonal expression that can be translated as "we have (to)", referring to the general "we", as long as the context allows it.
Because "il" is an impersonal, dummy subject here. It doesn't mean "he", it means "it", as in "it is necessary (to)...". And it can be translated as "we have (to)...".
Why the "il faut" construction and not eg. "nous devons"?
I though "il faut" was supposed to be impersonal, but in Duolingo, nearly all "WE have to..." phrases are fornulated with "il faut".
No, it can't. You can definitely say un mur plus grand, but it means a bigger wall
"We have to build a higher wall" uses the subjonctif doesn't it?
So it would be "il faut que nous construisions un mur plus haut", right?
Second question, why is it "un mur plus haut" instead of "un plus haut mur"?
Your sentence with the subjunctive is a correct option.
As for "mur plus haut" versus "plus haut mur", grammar sites say that "haut" is a BANGS adjective, so we would expect it before the noun, but I've found examples of both "plus haut mur" and "mur plus haut", and I wonder if the French here can actually be thought of as "we have to build a wall higher".
On the Google Ngram graph at the other end of the following link, "haut mur" is shown as more common than "mur haut", whereas "mur plus haut" is shown as more common that "plus haut mur":
As PeaceJoy mentioned your sentence is correct, but to explain a little the rule is that the construction "il faut que..." is followed by the subjunctive but "il faut..." is followed by the normal indicative mood. Your example is closer to the English version which specifies "we" will be building the wall, while the "il faut" version is a more general and impersonal statement which doesn't specify who will be doing it. If only Donald spoke French....
Good additional explanation, though I'll just point out that the infinitive isn't technically indicative.
(Some grammar explanations say that the infinitive itself is a grammatical mood, like the indicative and the subjunctive, and some say it isn't considered an example of a mood.)
I put "Nous devons construire un mur plus haut" and it was accepted.
I seems that it should be. "Bâtir le mur" and "construire le mur" are both fairly common in French-language publications, though the latter is perhaps slightly more common, if Google search results can be believed.
Isn't "haut/e" a BANGS adjective? Shouldn't it precede its noun? can somebody please explain?
Why is once: "Je dois prévoir ça - I have to plan that" and later "Je dois prévoir ça - I have to allow for that." Both times was declared flase?
It should be translated it is necessay to build a higher wall if youare going tp say il faut que. We have to is literally nous devons.