1)
Why does it say "Lachen"?
1.
Rashi: It is an expression of a Shevu'ah.
2)
Why does it say that the youths will fall in the streets?
1.
Malbim: They will not fall on the battlefield.
3)
What is the meaning of "Yidamu"?
1.
Rashi: They will be destroyed. It is also an expression of Demamah (silence), for no voice is heard in a desolate city.
2.
Radak: They will be cut off [Malbim - and not be valiant]. The same applies to "Tidamu ba'Avonah" (51:6).