For words that have only 3 letters, there's no 4th letter. For example with the word “insert”, there aren't any extra letters Later on and therefore must be the word "insert". The word “addict” is from the BIP39 wordlist however , you would have utilised “addi” for “addict”. Mnemonic words https://bookmarkgenius.com/story18793281/helping-the-others-realize-the-advantages-of-bip39-seed-phrase-mnemonics-generator