1. They started building this house a month ago.
-> This house has been built since last month
2. “Would you mind if I took you to the airport, Liz?” Jack said.
-> Jack offered Liz to take her to the airport
3. You sent me a lovely gift on my birthday. Thank you very much for it.
-> Thank you very much for the lovely gift  that you sent me on my birthday
4. The bad weather prevents us from going camping.
-> If the weather weren’t bad, we could go camping

Sentence transformation
1. They started building this house a month ago.
-> This house has been built since last month
2. “Would you mind if I took you to the airport, Liz?” Jack said.
-> Jack offered Liz to take her to the airport
3. You sent me a lovely gift on my birthday. Thank you very much for it.
-> Thank you very much for the lovely gift  that you sent me on my birthday
4. The bad weather prevents us from going camping.
-> If the weather weren’t bad, we could go camping