I've written to them three times but they haven't reply yet.
2. I've asked you again and again, but you've not done it yet.
3. I lent him $10 last month, but hw hasn't given it back yet.
4. He lost his pen a week ago, but he hasn't found it yet.

1. I've written to them three times. (not reply)
I've writen to them three times but they haven't replied yet
2. I've asked you again and again. (not do it)
I've asked you again and again but You haven't done it yet
3. I lent him $10 last month. (not give it back)
I lent him $10 last month  but he hasn't given it back yet
4. He lost his pen a week ago. (not find it)
He lost his pen a week ago but he hasn't found it yet