Learn Farsi: مطمئن نیستم.

While Farsi uses the impersonal enclitic structure for many physical and mental states (like being cold, sard-am ast), being 'sure' or 'certain' is treated as a regular adjective. You just use the standard verb 'to be' (hastam / nistam). You literally say 'I am not sure', just like in English.

How do you pronounce مطمئن? That letter in the middle looks unusual.

It is pronounced motma'en. The letter ئ (a hamza sitting on a base) represents a glottal stop, which is a tiny catch in your throat like the pause in the middle of 'uh-oh'. In casual everyday speech, people often smooth it out to sound like mot-ma-en.

If I want to say 'Are you sure?', do I just change the verb?

Yes! You change نیستم (I am not) to the positive 'you are' form, هستی (hasti). With a questioning tone, it becomes مطمئن هستی؟ (Are you sure?). In spoken Farsi, the verb usually attaches directly to the word, becoming مطمئنی؟ (motma'eni?).

Can I attach the verb directly to مطمئن to say 'I am sure'?

Absolutely. In the affirmative, it is extremely common to attach the short 'to be' suffix: مطمئنم (motma'en-am). However, because this sentence is negative, the negative verb نیستم must stand alone as a separate word.