Sometimes I catch myself using that phrase when a conversation turns a bit sharp; it's a compact way to say, 'You must be underestimating me.' In plain English, 'Do you think I have forgotten?' roughly translates to Indonesian as 'Kamu kira aku lupa?' or more politely 'Apakah kamu berpikir aku sudah lupa?' I use it when I want to call out someone who assumes I’ve let something slip — a deadline, a promise, or an important detail. Tone matters: said with a smile it feels cheeky; said coldly it can sting.
If you want to soften it, try adding a reason or a memory: 'Kamu kira aku lupa? Aku masih ingat ketika...' or in English, 'Do you think I have forgotten? I remember bringing it up last week.' In a professional or polite setting swap to 'Apakah Anda pikir saya sudah lupa?' or 'I haven't forgotten' to avoid sounding defensive. I tend to reach for this line when I need to assert that I’ve been paying attention, and it usually gets the point across—sometimes with a laugh, sometimes with raised eyebrows, but always with a little personal satisfaction.