You can use either 좋지 or 좋잖아. The nuance is different. A: 선물 좋아해? B: 좋지 (of course) I like it B: 좋잖아 I like it. (You know this) *Adjective/Verb + 잖아(요): This grammar pattern is used when both the speaker and listener(s) know a mutual fact. This can't always be translated directly to a particular English phrase or may sound awkward directly translating, since saying the other person already knows something can be known just on the context. Examples: 내 말이 맞잖아요. You know what I'm saying is right. 이게 아니잖아요. This isn't it (you know this). 그렇게 말하면 내 마음이 아프잖아. You know talking like that makes my heart hurt. A: 장미꽃 좋아하세요? Do you like roses? B: 네. 예쁘잖아요. Yes, they're pretty (you know this). *Adjective/Verb + 지(요) : This grammar pattern can be used in various ways: Used like a tag question in English, mini questions we put at the end of a sentence and invite the listener to agree with you, usually on some fact or topic you both know about. If you are using this pattern like a tag question, you must use a rising intonation when speaking. For giving information about something. This pattern uses a falling intonation when spoken. If used as a reply to someone else, can give a slight feeling of "of course ~." For non tag question it used when you know the other person knows something, especially when asking yourself a question or having someone confirm or recheck a fact. The intonation is different from when asking a tag question(see the audio examples below). Examples: 1)Used with Tag Questions 오늘은 춥지요? It's cold today, isn't it? polite informal form 내일 직장에 안 가지? You're not going to work tomorrow, are you? polite informal form 2)Used to Give Information A: 그 사람 알아? Do you know that person?" B: 모르지. (of course) I don't know him.