In case (something goes wrong): lỡ / nhỡ

Lỡ means "in case" or "what if", and it's always about something going wrong. Put it before the bad thing that might happen: Anh mang dù theo đi, lỡ trời mưa. Take an umbrella, in case it rains. Em đi sớm, lỡ kẹt xe. I'm leaving early in case there's traffic. In Hà Nội the word is nhỡ: nhỡ trời mưa.

At the start of a question, with thì sao (then what) at the end, it voices a worry: Lỡ không ai tới thì sao? What if nobody comes? You'll also hear lỡ như and lỡ mà, which mean the same.

Nếu (if) is neutral. Lỡ is only for what you'd rather avoid, and it's casual. The neutral "in case", for writing too, is phòng khi: Mang theo ít tiền phòng khi cần. Take a bit of money in case you need it.

Before something that has already happened, lỡ means "by accident": Em lỡ làm bể ly rồi. I've accidentally broken a glass (in Hà Nội, làm vỡ cốc). A possible event after lỡ means "in case"; a done one, often with rồi, was an accident.