Why will our mouths be full of laughter and our tongues full of song?
Malbim: Now, the laughter is in our hearts, and it cannot fill our mouths. Then, our mouths will be full of it. Now, our tongues sing in a dream, without motion. It is only in potential, but not in practice. Then, they will be full of song.
Why will they say "Higdil Hashem La'asos Im Eleh"?
Radak: They will be astounded at the wonder [that Hashem did for Yisrael].
Malbim: They will say so "Az" (then); it will be a new matter for them, as if it happened only then. For us, it is not new. We already rejoiced in the past, when we were like dreamers. We saw in a vision and rejoiced over what Hashem did for us, for it was clear to us, as if it already occurred.
