For words that only have 3 letters, there isn't any 4th letter. One example is with the word “insert”, there won't be any a lot more letters afterwards and so needs to be the word "include". The word “addict” is within the BIP39 wordlist but you would've used “addi” for https://ztndz.com/story22289576/5-essential-elements-for-bip39-explained