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Screw them down (check for wires, old gas pipes (I've 'found' several!) and heating pipes etc with tung tite or similar board screws. Knock in any nails then plain off any super high spots, most pine boards cup eventually, powerplaner makes light work of it. 3-5mm ply is enough to reduce the cupping effect that you'll see in the carpet. Hardboard is barely much different from cardboard, stops carpet going down any of the cracks but you'll still see and feel the cups/warped boards through.
Rip whole lot up and 25mm T+G chipboards if you want to achieve perfect level but its always more work than you think.
BrickMan
My hallway is getting carpeted in a month, but some of the boards are very short/warped/knackered from historical rot. What's the best method of getting a good solid sub-floor? T&G chipboard, ply, or replacement boards It's more or less the whole upper landing, about 3m²