For words that have only three letters, there isn't a 4th letter. For instance with the word “add”, there isn't any additional letters afterwards and thus must be the word "include". The word “addict” is in the BIP39 wordlist however , you would have used “addi” for “addict”. Mnemonic words https://bookmarkrange.com/story20392318/the-fact-about-bip39-seed-phrase-word-list-that-no-one-is-suggesting