How Electrum knows to validate BTC private key?

[Unsaint32]

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?   

[OmegaStarScream]

Whether the private keys is valid or not is totally based on what you entered and whether it follows the proper format or not.

[bob123]

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.