Can you tell me the difference in meaning betwenn the following pairs of sentences?

1. If I had the money, I'd buy a new car.
If I have the money, I'll buy a new car.

2. The house was very isolated.
The house was very secluded.

3. Give me a hand with this bag, will you?
Could you give me a hand with this bag, please?

Thank you for you help.