Else if thou wilt not let my people
`am  (am)
a people (as a congregated unit); specifically, a tribe (as those of Israel); hence (collectively) troops or attendants; figuratively, a flock -- folk, men, nation, people.

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go
shalach  (shaw-lakh')
to send away, for, or out (in a great variety of applications)

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behold I will send
shalach  (shaw-lakh')
to send away, for, or out (in a great variety of applications)

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swarms
`arob  (aw-robe')
a mosquito (from its swarming) -- divers sorts of flies, swarm.

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of flies upon thee and upon thy servants
`ebed  (eh'-bed)
a servant -- bondage, bondman, (bond-)servant, (man-)servant.

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and upon thy people
`am  (am)
a people (as a congregated unit); specifically, a tribe (as those of Israel); hence (collectively) troops or attendants; figuratively, a flock -- folk, men, nation, people.

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and into thy houses
bayith  (bah'-yith)
a house (in the greatest variation of applications, especially family, etc.)

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and the houses
bayith  (bah'-yith)
a house (in the greatest variation of applications, especially family, etc.)

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of the Egyptians
Mitsrayim  (mits-rah'-yim)
Mitsrajim, i.e. Upper and Lower Egypt -- Egypt, Egyptians, Mizraim.

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shall be full
male'  (maw-lay')
a primitive root, to fill or (intransitively) be full of, in a wide application (literally and figuratively)

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of swarms
`arob  (aw-robe')
a mosquito (from its swarming) -- divers sorts of flies, swarm.

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of flies and also the ground
'adamah  (ad-aw-maw')
soil (from its general redness) -- country, earth, ground, husband(-man) (-ry), land.

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whereon they are

Parallel Verses

New American Standard Bible
"For if you do not let My people go, behold, I will send swarms of insects on you and on your servants and on your people and into your houses; and the houses of the Egyptians will be full of swarms of insects, and also the ground on which they dwell.

King James Bible
Else, if thou wilt not let my people go, behold, I will send swarms of flies upon thee, and upon thy servants, and upon thy people, and into thy houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground whereon they are.

Holman Christian Standard Bible
But if you will not let My people go, then I will send swarms of flies against you, your officials, your people, and your houses. The Egyptians' houses will swarm with flies, and so will the land where they live.

International Standard Version
But if you don't let my people go, I'll send swarms of insects upon you, your servants, your people, and your households. The houses of Egypt—and even the ground on which they stand—will be filled with swarms of insects.

NET Bible
If you do not release my people, then I am going to send swarms of flies on you and on your servants and on your people and in your houses. The houses of the Egyptians will be full of flies, and even the ground they stand on.

GOD'S WORD® Translation
If you will not let my people go, I will send swarms of flies on you, your officials, your people, and your houses. The homes of the Egyptians will be filled with flies, and even the ground outside will be covered with them.

King James 2000 Bible
Else, if you will not let my people go, behold, I will send swarms of flies upon you, and upon your servants, and upon your people, and into your houses: and the houses of the Egyptians shall be full of swarms of flies, and also the ground on which they are.

Links

Exodus 8:21
Exodus 8:21 NIV
Exodus 8:21 NLT
Exodus 8:21 ESV
Exodus 8:21 NASB
Exodus 8:21 KJV