Here are the mistakes in each sentence along with the corrections:

1. **We have learnt a lot about the culture of ethnic groups since we arrive there.**
- Mistake: D. arrive
- Correction: arrive → arrived

2. **He just has given a performance that was both touching and brilliant.**
- Mistake: A. just
- Correction: just → has just

3. **Going snorkelling was the most exciting activity I have ever experienced.**
- Mistake: B. was
- Correction: was → is (if considering present relevance)

4. **Did you ever felt helplessness when being unable to put up tents?**
- Mistake: A. Did
- Correction: Did → Do (if considering present relevance)

Please let me know if you need further assistance!