With frogs - Some months appear to have elapsed between this and the former plague, if the frogs made their appearance at the usual time, that is in September. The special species mentioned here is of Egyptian origin. This plague was, like the preceding, in general accordance with natural phenomena, but marvelous both for its extent and intensity, and for its direct connection with the words and acts of God's messengers. It had also apparently, like the other plagues, a direct bearing upon Egyptian superstitions. There was a female deity with a frog's head, and the frog was connected with the most ancient forms of nature-worship in Egypt.

If thou refuse - Nothing can be plainer than that Pharaoh had it still in his power to have dismissed the people, and that his refusal was the mere effect of his own wilful obstinacy.
With frogs - צפרדעים tsepardeim. This word is of doubtful etymology: almost all interpreters, both ancient and modern, agree to render it as we do, though some mentioned by Aben Ezra think the crocodile is meant; but these can never weigh against the conjoint testimony of the ancient versions. Parkhurst derives the word from צפר tsaphar, denoting the brisk action, or motion of the light, and ידע yada, to feel, as they seem to feel or rejoice in the light, croaking all the summer months, yet hiding themselves in the winter. The Arabic name for this animal is very nearly the same with the Hebrew zafda, where the letters are the same, the ר resch being omitted. It is used as a quadriliteral root in the Arabic language, to signify froggy, or containing frogs: see Golius. But the true etymology seems to be given by Bochart, who says the word is compounded of zifa, a bank, and rada, mud, because the frog delights in muddy or marshy places; and that from these two words the noun zafda is formed, the re being dropped. In the Batrocho myomachia of Homer, the frog has many of its epithets from this very circumstance. Hence Λιμνοχαρις, delighting in the lake; Βορβοροκοιτης, lying or engendering in the mud; Πηλευς, and Πηλβατης, belonging to the mud, walking in the mud, etc., etc.
A frog is in itself a very harmless animal; but to most people who use it not as an article of food, exceedingly loathsome. God, with equal ease, could have brought crocodiles, bears, lions, or tigers to have punished these people and their impious king, instead of frogs, lice, flies, etc. But had he used any of those formidable animals, the effect would have appeared so commensurate to the cause, that the hand of God might have been forgotten in the punishment; and the people would have been exasperated without being humbled. In the present instance he shows the greatness of his power by making an animal, devoid of every evil quality, the means of a terrible affliction to his enemies. How easy is it, both to the justice and mercy of God, to destroy or save by means of the most despicable and insignificant of instruments! Though he is the Lord of hosts he has no need of powerful armies, the ministry of angels, or the thunderbolts of justice, to punish a sinner or a sinful nation; the frog or the fly in his hands is a sufficient instrument of vengeance.

And if thou refuse to let [them] go, behold, I will smite all thy borders with (a) frogs:
(a) There is nothing so weak that God cannot use it to overcome the greatest power of man.

And if thou refuse to let them go,.... Will not obey the orders:
I will smite all thy borders with frogs; he gives him warning of the blow before he strikes, which shows his clemency and goodness, his patience and longsuffering; and this he did, not only that he might have time and space for repentance, and thereby avoid the blow; but that when it came, he might be sensible it was not by chance, or owing to second causes, but was from the Lord himself.
I will smite all thy borders with frogs: fill the whole land of Egypt with them, to the utmost borders thereof on every side. Some (q) say the word signifies a large Egyptian fish, which in the Arabic tongue is called Altamsach, that is, a crocodile, with which the Nile abounded; but such a creature could not invade and attack them in the manner as is after related.
(q) R. Chananeel apud Abendana, and some in Aben Ezra in Ioc.

I will smite all thy borders with frogs--Those animals, though the natural spawn of the river, and therefore objects familiar to the people, were on this occasion miraculously multiplied to an amazing extent, and it is probable that the ova of the frogs, which had been previously deposited in the mire and marshes, were miraculously brought to perfection at once.

All thy borders - All the land that is within thy borders.