can i use bicycle 9 speed rear deraileur for 8 speed cassette?

You wont have any problem as long as the cassette and shifters are the same number of speeds. There is plenty of room on a 10 speed derailleur for a 7-8 speed chain. The inside width of both chains are the same the outside width is smaller on a 10 speed chain.

Yes you can use a 9-speed derailleur on an 8-speed cassette. All Shimano derailleurs use the same pull ratio, which is 2:1. So pulling an inch of chain moves the derailleur a half inch. Since 9-speed and 8-speed cassettes have different distances between gears, the shifters for each will pull different amounts of cable between detents. So the shifters, specifically the rear shifter, is the part that must match the cassette type.

The M430 will work with your 8 speed cassette. You have to consider primarily whether the model you choose will accommodate the range of teeth on the cassette. In this case the M430 is rated for a 34 tooth largest cog. That said you will have to set the limit screws right to be sure the chain doesn't get pushed into the spokes. You will also need a Shimano 8-speed shifter to ensure the shifts index correctly.

The rear derailleur does not control the number of speeds. That is the job of the shifter, cassette, and chain. So if you use 8 speed shifters with an 8 speed cassette and chain you can use a "9 speed" derailleur. I am currently using a 9 speed Deore rear derailleur with 8 speed shifters and cassette on my touring bike. It works flawlessly

Yes, you need to adjust it, specially so that the chain doesn't drop into the spokes because it will cut through them. The shifting is dictated by the shifters and you still will have 8 indexed positions that match the distance between your gears, so the displacement of the cassette will be correct.

Derailleur would work, 9 speed shifter would not.

I believe some 10 speeds derailleurs have a different cable pull, so it isn't universally true.

yes derailleur doesn;t know the difference

the shifters do though

wle

yes it should work you may just have to tune it up