Learn Arabic (Modern Standard): بما أننا في هذا الوادي القاحل، يجب أن نحفظ بعض المؤونة. - Since we are in this barren valley, we must save some provisions.

The phrase بما أنّ means "since" or "given that". Because أنّ (that) is from the "Inna and her sisters" family of words, it must be followed directly by a noun or an attached pronoun. To say "since we", we attach the pronoun نا (we/us) to get بما أننا. You cannot use the independent pronoun نحن after it.

Why do both الوادي and القاحل have the definite article ال after the word هذا?

To say "this valley", the noun after the demonstrative هذا (this) must take ال. If it didn't, هذا واد would mean "This is a valley". The word القاحل (barren) is an adjective describing the valley, so it must match الوادي in definiteness and also takes ال.

Why is the verb نحفظ (we save) in the subjunctive mood here?

It comes directly after أن (that/to) in the phrase يجب أن (we must / it is necessary that). The particle أن forces the following present tense verb into the subjunctive mood, meaning the standard verb نحفظ takes a fatha on its final letter instead of the usual damma.

What grammatical structure is used for بعض المؤونة (some provisions)?