Here are the correct forms for each exercise:

**Ex 5: Write the correct form or tense of the verbs in brackets.**

1. She **has not set** foot in Ho Chi Minh City since she graduated from the University of Economy.
2. It’s hard for me to **concentrate** on working in such a noisy office.

**Ex 6: Find a mistake in the four underlined parts of each sentence and correct it.**

1. I used to listen to music **while** I was riding my bicycle to school.

The mistake is in part "during" which should be corrected to "while".