Is the monthly return the bank quoted on my certificate guaranteed? Will it stay that way to maturity?

It depends on the rate type. If the rate type is fixed, the rate is set in the contract for the whole term, and the monthly amount will not change as long as the certificate is not broken. If the certificate is stepped with a published schedule, the rates are also in the contract, but they change from year to year as scheduled. If the return is variable, the rate is tied to a reference rate plus the bank's margin, and the figure you were told is computed at today's rate; if the reference changes, the new rate applies after the current payout period, so the amount can rise or fall. In Islamic products, the figure is usually an expected profit settled on the bank's actual results. Usually, the original amount in a conventional certificate comes back at maturity (a certificate pledged against a loan stays held until repayment), but breaking early can return less than it. No one can tell you where reference rates will go, so if a steady figure matters to you, weigh a fixed type against a variable one yourself. Check the key-facts statement for the rate type, the reference rate and margin if variable, and the break-penalty table.