إِنْ صَلَحَ الصِّمَامُ قَبْلَ الْمَسَاءِ عَادَ الْمَاءُ، وَإِلَّا أُعْلِنَ مَوْعِدٌ جَدِيدٌ.
If the valve is repaired before evening, the water will return; otherwise, a new time will be announced.
In marks a condition, so the predicted return is tied to the repair. Wa-illā identifies the alternative procedure. Treating this as equivalent to 'Water will certainly return before evening' would erase the condition and turn an estimate into an unconditional assurance.
Rhetorical precision requires attention not only to whether a statement is true or false, but also to how firmly it is asserted. The essential condition should not be sacrificed for brevity.