I don’t know why so many students make this error, but I see it over and over again.
writing defiantly when you mean definitely
Definitely means absolutely, clearly, obviously– all those good things. Defiantly means disobediently, going against something. The root word is defy. This is one of those errors that significantly changes the meaning of your sentence.
Spellcheck will not pick up this switch, so when proofreading, stop at every “definitely” to double check.