Why won't 92 Isuzu manual transmission go gear while it is running?

hi

no.1 could be your clutch master or slave cylinder fluid has run out by leaking out the cylinders

2.not enough presssure in hydraulic system to enable clutch to disengage when clutch pedal is depressed.

3.faulty transmission -no oil in gearbox

4. broken components in box like fork or selector shaft lock pins or worn out synchros(which will cause it to grind or grate when engaging gears)

5.also check gear lever ,if bottom end that is below the ball pivot ,the part that pushes and pull the selectors is not worn or broken -

6.if the gear feel like it clicks in when it is engaging but wont move then check clutch plate is not worn (burnt out )or broken -

there are many more checks to do , these are the the most common faults.