separate
There is a rat in separate. Remember the rat.
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Wrong: seperate
Right: separate
The middle three letters spell rat.
The trick
There is a rat in sep-a-rate. The middle three letters spell rat, and the vowel before it is an A.
This is one of the most-misspelled words in English in basically every published ranking, and the rat is the reason anybody ever gets it right.
Now go find the rat.