when laying a neew carpet and the floorboards are uneven. what would be the best thing to put under the carpet?

you could put the green padded underlay,then normal underlay as this will help to take away the uneven surface

If floorboards are uneven then ply liner should be used to level and bridge uneveness. WBP ply is best as is moisture resistant. Thickness of ply would be dependant on how uneven floorboards are anything between 4 and 6 mm ply.

Hi the best thing to do is tack down hardboard sheeting (oil tempered is best) but standard is ok. then your ready to lay your underlay and carpet.

Minimum 4mm ply then a quality underlay but make good floorboards first

an underlay, or 3mm hardboard and an underlay, or 6mm ply and an underlay. All depending on how bad your floor boards are. If they are uneven by more than 6mm i would replace the floor.