Loans
Recording a loan, and paying it with the interest split out.
Loans → Loans list → New loan. Enter the principal, the annual rate, the date the funds arrived, the first payment date and the term.
Elvira posts the receipt: money into the bank, a liability owed: and builds an amortisation schedule.
Paying an instalment
Record the payment from the loan. It is split into principal and interest for you, using the schedule. If your agreement states a different instalment from the computed one, enter the contract figure and the split follows from it.
Current portion
What falls due within twelve months is reclassified out of long-term liabilities, and re-measured as the loan is paid down.
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Open Loans → Loans list to see principal, rate, term and what is still outstanding.

The register. Recording a payment from here splits it into principal and interest for you.