Here are the rewritten sentences as per your requirements:

1. Young individuals **must** show respect towards the older generation.

2. Teenagers are encouraged to spend less time on social media, such as Facebook or TikTok.

3. Children **must not** argue stubbornly with their parents.

4. Nowadays, it is not necessary for young people to follow in their parents' footsteps.

5. How long is it since you started playing tennis?
- **When did you start playing tennis?**

6. I have **never** watched such an exciting football match.
- **Ever since I started watching football, I have never seen a match as exciting.**

7. I have not met each other often since we left school.
- **The last time we met was when we left school.**

8. I **began** teaching English at Nguyen Hue High School 17 years ago.

1. Young individuals **must** show respect towards the older generation.

2. Teenagers are encouraged to spend less time on social media, such as Facebook or TikTok.

3. Children **must not** argue stubbornly with their parents.

4. Nowadays, it is not necessary for young people to follow in their parents' footsteps.

5. How long is it since you started playing tennis?
- **When did you start playing tennis?**

6. I have **never** watched such an exciting football match.
- **Ever since I started watching football, I have never seen a match as exciting.**

7. I have not met each other often since we left school.
- **The last time we met was when we left school.**

8. I **began** teaching English at Nguyen Hue High School 17 years ago.