Tafsir of Al-Muzzammil 73:15-16

"Indeed, We have sent to you..."

The address is to the people of Mecca.

"A witness over you" He will testify against you on the Day of Resurrection regarding your disbelief and your denial.

If you ask: Why was the Messenger mentioned indefinitely (as a noun) and then defined (with the definite article)? I say: Because He meant: "We sent to Pharaoh a messenger." When He repeated it—and he was already mentioned—He added the definite article (al-) to point to that specific individual.

"A grievous [punishment]" (wabīlan) Heavy and severe. It is derived from their saying: "Wabīl and kham fodder," meaning it is not easily digested due to its heaviness. Al-wabīl also refers to a thick staff, and from this comes al-wābil (the downpour) for heavy rain.

"Then how will you protect yourselves, if you disbelieve, from a Day that will make the children white-haired? The heaven will break apart therefrom; His promise is ever fulfilled."