High-Voltage Pulse Measurement Using Just Resistors for oscilloscope measurment
As others have said, 10's of nano seconds is high frequency. If you wanted a purely resistive divider and chose values of lets say \$R1 = 10 M\Omega\$ and \$R2 = 10k\Omega\$ which would create a ~1000:1 ratio. Unfortunately, even just a 1pF parallel capacitance which is well within the possibility of stray capacitance, would cause your Vin and Vout to look like this:









simulate this circuit &ndash; Schematic created using CircuitLab

(Note that C2 out to be 1pF NOT 1uF. The simulation was done with C2=1pF)

A better option, if you only need to measure a short pulse, would be a purely capacitive divider. Of course if you need to measure DC this would not be an option. Going this route you would be concerned with series inductance which would be hard to mitigate using discrete components. Measuring high voltage pulses this fast is far from trivial. That is why high voltage scope probes with 50-100Mhz bandwidth are so expensive. You could go the route of making a capacitive divider where the physical construction of your probe gives you the capacitance needed (see this paper). This makes minimizing the inductance much more feasible.