וַיֹּ֨אמֶר יְהוָ֜ה אֶל־מֹשֶׁ֗ה נְטֵ֤ה יָֽדְךָ֙ עַל־הַשָּׁמַ֔יִם וִ֥יהִי חֹ֖שֶׁךְ עַל־אֶ֣רֶץ מִצְרָ֑יִם וְיָמֵ֖שׁ חֹֽשֶׁךְ׃
여호와께서 모세에게 이르시되 하늘을 향하여 네 손을 들어서 애굽 땅위에 흑암이 있게 하라 곧 더듬을만한 흑암이리라
Rashi on Exodus
וימש חשך signifies and it (the darkness mentioned in the preceding phrase, ויהי חשך) shall darken for them the natural darkness to a higher degree than the darkness of night: i. e. the darkness of night shall become even more black and dark.
Rashbam on Exodus
וימש חשך, same as ויאמש חשך. This construction with variable spelling is similar to Isaiah 13,20 ולא יהל שם ערבי meaning the same as ולא יאהל שם ערבי, “no Arab shall pitch his tent there.” By remembering this missing letter א, we understand the meaning of the line as “the darkness of the (previous) night will continue and intensify so that there will be no light for three days.”
Sforno on Exodus
נטה ידך על השמים, “in the direction of the atmosphere referred to as “heaven.” We have explained this in connection with Genesis 1,7.