For words that have only three letters, there isn't a 4th letter. For example with the word “incorporate”, there isn't any extra letters afterwards and thus must be the word "add". The word “addict” is in the BIP39 wordlist however, you would have used “addi” for “addict”. A seed phrase, https://scrapbookmarket.com/story18807434/5-essential-elements-for-bip39-explained