Here's every excuse you could ever need! Let’s go through all of them. You can start with:
Sorry, I'd love to help you but ...
... I'm a bit tied up.
... I'm otherwise engaged.
… my back's playing up again.
... I'd be completely out of my depth.
... something else has cropped up.
... life's a bit hectic right now.-busy
... I'm in the middle of something urgent.
... it's out of my hands.-Not within one's control.
... I've got a lot on my plate.
... I've just got to pop out for a moment.
... I'm totally snowed under.-having too much to do
... it's an awkward moment.
... I can't help for, er, personal reasons.
1. -Could you carry my bag for me? It's too heavy for me.
-Sorry, I'd love to help you but my back's playing up again.
2. -Could you drive me to the shopping center ... and wait for me while I'm shopping?
-Sorry, I'd love to help you but I'm in the middle of something urgent.
3. -Could you clean my windows?
-Sorry, I'd love to help you but I'm totally snowed under.
.4 -Can I come to your house/flat for a meal ... and bring a few of my friends?
-Sorry, I'd love to help you but I'm otherwise engaged.
5. -Could you look after my pet snakes for a few weeks?
-Sorry, I'd love to help you but… I can't help for, er, personal reasons.
6. -Could you go to the airport to welcome my visitors?
-Sorry, I'd love to help you but... something else has cropped up.