لَيْتَ الشَّبَابَ يَعُودُ يَوْمًا
If only youth returned one day.
Layta was primarily assigned to the meaning of wishing, so it is the primary particle of tamannī. It behaves grammatically like inna: its noun is accusative, and its predicate occupies the nominative position.
لَيْتَنِي حَاضِرٌ
If only I were present.
The pronoun -nī functions as layta's noun and ḥāḍirun as its nominative predicate. Layta strongly signals the construction of a wish, while context explains the nature of what is wished for.