how to record a part payment of the principle and interest payment of an loan account?
this is private loans not bank loans
Any payment can be split into multiple lines. Use Split button for that.
This way you can split the amount and allocate individual portions to separate accounts. In your case, you would allocate something to liability account that represents loan and the interest portion should be allocated to interest expense account.
What about the long-term interest loan, example car loan or building at 10 year.