In some cases they may be used interchangably. "Mend" has to do with things clearly broken or torn (clothes, shoes, bones, sometimes plates and even behaviour - "to mend one's ways"). Mend can also work as an intransitive verb - actually with a reflexive meaning - which repair cannot do. Repair, as Mohsen said, is usually applied to technical things, to restore them to their original state. This does include instruments: "instrument repair". This word would have the most general usage - "to repair clothes" is definitely possible - but it wouldn't be used for human things (behaviour, relationships, hearts, bones etc) whereas "mend" would. In many cases, you'll find the meanings do overlap, as well as with the word "fix", although "fix" tells me you're working directly on the error or damage.

We use Mend for something like torn clothes! and Repair for machines and Fix for instruments also we have amend in legislation! ------------------------------------------------------- Am I right?

first one is correct