Why does my credit card charge interest even when I pay before the due date?

The likely reason is that you lost your grace period. The grace period, the window where new purchases don't accrue interest, only applies if you paid your previous statement balance in full. Once you carry a balance, most issuers charge interest on new purchases from the day you make them until you pay the whole balance for two consecutive cycles to reinstate the grace period.

So paying 'before the due date' isn't enough if you're paying less than the full statement balance. Another cause is a cash advance, which has no grace period and starts accruing interest immediately. To stop interest entirely, pay the full statement balance every month. Check your statement's 'interest charge calculation' section to see exactly what's accruing.