Here are the corrections for the mistakes in the sentences:

4. The parachute jump was **an** exhilarating experience **for** the boy. He was so excited.
- **Correction:** "a(n)" should be "an" (correct form).

5. Mary had a pleasant experience putting up a tent in the rain.
- **No correction needed.**

6. He felt **helplessness** when he couldn't protect himself from bullying.
- **Correction:** "helplessness" should be "helpless" (correct word form).

7. He studied German in college and practiced it regularly since then.
- **Correction:** "practised" should be "practiced" (American English spelling if consistent).

8. My friends have traveled to Asia last year and explored many countries in the region.
- **Correction:** "travelled" should be "traveled" (American English spelling if consistent).

9. I finished reading a book yesterday, and I already started a new one.
- **Correction:** "and" should be "but" (to connect contrasting ideas).

10. She lived in London for a year and has **missed** the city ever since she moved away.
- **Correction:** "miss" should be "missed" (correct tense).

The sentences should read:

4. The parachute jump was **an** exhilarating experience **for** the boy. He was so excited.

6. He felt **helpless** when he couldn't protect himself from bullying.

7. He studied German in college and **practiced** it regularly since then.

8. My friends have **traveled** to Asia last year and explored many countries in the region.

9. I finished reading a book yesterday, **but** I already started a new one.

10. She lived in London for a year and has **missed** the city ever since she moved away.