Não tenho certeza

You can use it alone as a complete sentence. You can also follow it with de que or se. For example, Não tenho certeza se ele vem means 'I'm not sure if he's coming.' It’s like a Lego brick. You can snap it onto almost any thought. Just remember to sound slightly hesitant when you say it. A little shrug of the shoulders helps too!

Use it when a friend asks for directions you barely remember. Use it in a meeting when you need to double-check a number. It works perfectly at a restaurant if you aren't sure what a dish is. It's great for texting when you might be late. Basically, use it anytime your confidence isn't at 100%.

Don't use it if you definitely know the answer but are just being shy. Avoid it if someone asks 'Do you love me?' unless you want a very long talk. It might sound too cold in a very emotional moment. If you are 100% sure, use Tenho certeza. Don't use it to dodge a simple 'yes' or 'no' question forever. People might think you are being indecisive.

Portuguese speakers value politeness and 'saving face.' Saying Não tenho certeza is softer than a blunt Não sei (I don't know). It implies that you *might* know soon. It leaves the door open for further conversation. In Brazil, people often prefer being vague over being wrong. It’s a social safety net that keeps conversations flowing smoothly.

You will often hear Não tenho muita certeza (I'm not very sure). In casual speech, people might say Não tenho certeza absoluta. That one means 'I'm not absolutely sure.' If you want to sound more formal, you can say Não estou certo disso. But honestly, the standard version is a winner everywhere. It’s the 'white t-shirt' of Portuguese phrases—it fits every occasion.