Here are the completed sentences:

1. I find collecting birthday cards interesting.
I'm interested in **collecting birthday cards**.

2. She likes socialising with friends more than watching TV.
She prefers socialising **to watching TV**.

3. If you don't study hard, you won't pass the mid-term test.
Unless you **study hard**.

4. Please help me with this exercise!
Would you mind **helping me with this exercise**?

5. If you get too attached to your smartphone, you won't have time for your family.
Don't get **too attached to your smartphone**.

6. Going fishing in the rural areas is really enjoyable.
I really enjoy **going fishing in the rural areas**.

7. He was kind of addicted to the Net, but now he isn't.
He used to **be kind of addicted to the Net**.

8. Why don't we take a break and relax for a while?
Let's **take a break and relax for a while**.