For words that only have three letters, there isn't a 4th letter. For example with the word “add”, there isn't any much more letters Later on and thus must be the word "insert". The word “addict” is inside the BIP39 wordlist however, you would've used “addi” for “addict”. Mnemonic words https://bookmarkzap.com/story18853468/the-2-minute-rule-for-bip39-all-words