Commentary for Exodus 7:27

ואם מאן אתה — AND IF THOU BE A REFUSER. מאן — The word has the same meaning as ממאן, refusing, (the Piel participle) only the difference is that it is a description of a person having reference to the action he always does (i. e. it is an adjective and the full phrase would be איש מאן) similar to שָׁלֵו “a man at ease” (Job 16:12); שָׁקֵט “a man at ease” (Jeremiah 48:11); וְזָעֵף [‎סר] “a man displeased” (1 Kings 20:43), whilst מְמאֵן merely describes the person as refusing at some particular moment.

נוגף, another word for makkeh, “strike, smite.”

ואם מאן אתה, "if you should refuse, etc." We learn from here that for a warning to be legally valid it must be delivered immediately preceding the sin.