Morning by morning means “every morning,” or “every day when the sun rose.” They gathered it means they collected the manna from the ground. Each as much as he could eat, or better, “as much as he needed” (Good News Translation). (See the comment at verse 16.)

But when the sun grew hot means, of course, “when the sun heated it” (Durham), for although the sun is always hot, its rays do not hit the ground directly until well after sunrise. Only when the sun had risen fairly high in the sky would the rays have hit the manna directly and melted it. So one may translate “when the sun rose high in the sky, it melted.” It melted refers to “what was left on the ground” (Good News Translation), for what was collected would have been protected from the sun.

Quoted with permission from Osborn, Noel D. and Hatton, Howard A. A Handbook on Exodus. (UBS Helps for Translators). New York: UBS, 1999. For this and other handbooks for translators see here .