An explanation of the negative interest is when you pay a premium for a cash flow which has a stated rate of interest.
You buy a note from a seller. The terms of the note are 100K loan at 8% interest for 1 year.
The face value of the note is $100K. You might buy the note (for whatever reasons) from the note-maker for $110K.
The payments remain at 8% interest. However, because you invested more, your real return is negative. You invested $110K and in return, you will receive monthly payments totaling $104,385. Your effective investment return is -9.5%
usually it is not this extreme, but it shows you the example.
This premium situation is common with bond sales.