لَيْتَ الشَّبَابَ يَعُودُ.
If only youth would return.
Layta presents a wish (tamannī), expressing desire even when attainment is remote. In contrast, an expression of hope may presuppose a realistically possible future event.
لَعَلَّ الْمُسَافِرَ يَصِلُ الْيَوْمَ.
Perhaps the traveler will arrive today; I hope so.
Here laʿalla can convey expectant hope (tarajjī). The key distinction lies not simply in future time, but in the speaker’s attitude: wishing versus anticipating an attainable outcome.