Learn Arabic (Modern Standard): مشى الرجل في الغابة حتى سقط من الإرهاق. - The man walked in the forest until he fell from exhaustion.

You are right that حتى often takes a present subjunctive verb for future goals. However, because this sentence describes an event that already concluded in the past (he walked and actually fell), we use حتى followed directly by the past tense verb سقط to express a realized past result.

Could we use بسبب (because of) instead of من (from) to say 'from exhaustion'?

Yes, بسبب الإرهاق is grammatically correct and means 'because of exhaustion.' However, using من is a very natural and common Arabic idiom to show the direct physical or emotional cause of a physical reaction, much like saying 'he fell from exhaustion' in English.

What type of word is الإرهاق (exhaustion)?

It is a verbal noun (masdar) from a Form IV verb. The base verb is أرهق (to exhaust or burden someone). The pattern for a Form IV verbal noun is to add an 'i' sound at the beginning and a long 'a' before the last letter, giving us إرهاق. With the definite article, it becomes الإرهاق.

Why does the verb مشى (he walked) end with a dotless ya (ى) rather than a regular alif (ا)?