A private key - basically - is just a random 256 bit number.
But what you import into electrum is most likely the private key in the WIF (= Wallet Import Format).
And the WIF key contains a checksum. So each wallet can easily check whether it is a valid private key or not by looking at the checksum (last 4 byte).
You can read more about the wallet import format here.
Topic: How Electrum knows to validate BTC private key? (Read 207 times)
Whether the private keys is valid or not is totally based on what you entered and whether it follows the proper format or not.
So I am learning Electrum wallet on my windows 10 desktop. To sweep my BTC from paper to the Electrum wallet, I type in my private key in the Electrum. I type one letter wrong, and the Electrum's "next" button remains grayed out. As soon as I type in the correct letter, the "next" button is now ready. My question is... How does Electrum know whether it is a valid key?
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