Past debt is not affected by current inflation.

However, current debt interest rates are affected by inflation expectations. Paradoxically, even though inflation can devalue past debt, it can't devalue future debt because creditors will demand nominal interest rates higher than inflation.

If real interest rates are 2%, if you say inflation will be 5%, they will demand 7%. If you say inflation will be 10% they will demand 12%. If you catch on and start increasing inflation every year, creditors will also catch on and start writing debt whose interest rate increases every year.

The nominal value of past debt is not affected by current inflation. Its real value decreases as inflation increases.

However, current debt interest rates are affected by inflation expectations. Even though inflation can devalue past debt, it can't devalue future debt because creditors will demand nominal interest rates higher than inflation.

If real interest rates are 2%, if you say inflation will be 5%, they will demand 7%. If you say inflation will be 10% they will demand 12%. If you catch on and start increasing inflation every year, creditors will also catch on and start writing debt whose interest rate increases every year. If you planned to devalue future debt using inflation, you would have to keep increasing inflation even faster than you did last time. Therefore it does make sense to speak of past debt nominally and future debt in real terms.