Commentary for Exodus 8:2

ותעל הצפרדע AND THE FROGS (lit. frog) CAME UP — Really there was only one frog, but when they struck at it, it was split into many swarms. This is a Midrashic explanation of the usage of the singular noun here (cf. Sanhedrin 67b; Exodus Rabbah 10:4). But as a literal explanation one must say that the swarm of the frogs is denoted by the singular form. Similar is, (v. 14) “and there was the כנם” — the swarm of insects, in old French pedulier; i. e. a swarm of lice. So, too, here, ותעל הצפרדע means: and there came up a grenouillière (old French) i. e. a swarm of frogs.

ותעל הצפרדע, “the frogs emerged from the river.” Even though the Torah uses the singular mode here, the meaning is that swarms of frogs came forth. We find something similar in Numbers 21,7 when the Torah describes the plague of snakes besetting the people as if it had been only a single snake, i.e. ויסר מעלינו את הנחש, “may He remove the snake(s) from us.”