Fix is often used to talk about repairing a vehicle or a machine. Mend is often used to talk about repairing holes in clothes, shoes, roofs or roads. It can also be used to talk about repairing anything that is broken.

fix

• Our car isn't working, but Dad will fix it soon.
• Oh, no! The computer got stuck again. Can you fix it?
• The little boy fixed the brakes on the bicycle by himself.

mend

• I think we'll need to mend the roof soon.
• His mother mended his trousers.
• Those shoes need to be mended.