أَلَا لَيْتَ الشَّبَابَ يَعُودُ يَوْمًا
فَأُخْبِرَهُ بِمَا فَعَلَ الْمَشِيبُ
If only youth returned one day, I would tell it what old age has done.
The poet does not expect his lost youth to return. The wish expresses regret for the passage of youth and the effects of aging.
al-shabāba is the accusative noun of layta. yaʿūdu is an indicative imperfect verb; the clause forms the predication within the wish. The following fa-verb expresses what the speaker would do if that wish were granted.
Fa-ukhbirahu is subjunctive in the answer to the wish; after causal fāʾ, an understood an governs its final fatḥah.