For words that only have three letters, there isn't any 4th letter. Such as with the word “insert”, there aren't any more letters afterwards and so need to be the word "include". The word “addict” is during the BIP39 wordlist however you would've employed “addi” for “addict”. One more benefit https://bookmark-group.com/story4377772/fascination-about-bip39-english-words