Car will only engage into a gear when car is off. the clutch feels like it's doing nothing and won't shift when engines on.

I'm assuming that this is a manual transmission.
Old truck drivers trick...... Push in the clutch just as you try to put it into gear. If you do it too soon, it may make a grinding noise. Practice..... Thr trick is to have the transmission just barely rotating as you're grabbing the gear. Older vehicles can develop shifting issues as the synchro rings get worn. Pretend you're a trucker and feel the shift happen.

If you have a manual gear box there are two things that you can check. First is the engine mounts especially the rear one to see if the rubber has broken away. Check all engine mounts for the same problem.. The second is a failing clutch or if you have a cable operated clutch it points to engine mounts. What appears to be happening is when you engage the clutch in first gear the torque reaction of the engine is rolling the gear box away from the clutch actuation arm or lever. The clutch slips ,the engine rolls back ,the clutch engages , the engine rolls and so you have a bad shudder. When you are moving the reaction is less as you move up through the gears . If all is good then have a qualified clutch and brake specialist do a check on the clutch itself.

Then there is a serious problem with the clutch. turn the engine off and engage a gear and try and roll the truck, do it feel like it is in gear? if not the clutch is either defective or the transmission input shaft is broken

You have one of these two things going on....Either the clutch master cylinder is internally bypassing or the clutch itself has gone bad. If you can pump the clutch and sometimes be able to shift, the master is bad. If the clutch pedal feels normal, then I'd suspect the actual clutch disc is worn out.
Your clutch has little to do with shifting with the engine off as nothing is turning and engaging the gears is not a problem. Have an experienced tech take a look at it if in doubt ...should take all of five minutes to determine what's wrong.

A high clutch pedal is good.
There should only be about a half inch of pedal freeplay, before you start to feel resistance.
So the clutch should not engage until it is almost fully released, near the top.
The makes shifting the easiest.

If the shifting is difficult when cold, then put the transmission in neutral and let the clutch out when you warm up the engine.
That will warm up the transmission oil.

Of course you could also check the transmission oil level and change it.
But your question makes me believe you may need more experience on a stick shift. Make sure you do the H pattern carefully, and do not get lazy and turn it into an X pattern.

clutch self adjuster is bad,you need to replase cabel/try orileys auto parts first,or dealer for cable.
tim

its a fulty gear box it needs to be repaire or replased there is nothing you can do that will make it any better brop it into the shop for fixing

Adjusted Clutch cable assembly under passenger side of vehicle. Had assistance from friend depressing clutch pedal and engaging shift lever. Problem went away

Exchange the transmission fluid with a synthetic lube for easier engagement. Shift to second first before shifting to first helps as well.

STOP Downshifting!
STOP Forcing gear change, watch the Engine RPMs and shift accordingly, HOMBRE's are as good as the operator's. Hombre's have dash shift indicators which illuminate when it's time to 'shift' and not before, This has nothing to do with gear oil either. If you've drained it, you best fill it immediately and make sure it's ample level. The heat these gears sustain in is inceredible, cooling lubricant is essential. Here's a tip, while engine is OFF, ignition key is engaged, dash is illuminated, depress clutch peddle, keep it depressed and walk through all gears with gear shift, shifting should be smooth and unpeded, including reverse. If resistance is experienced at one or more gears, gear alignment is off as a result of inproper gear shifting while driving. Continue to drive vehicle just shift slower until clutch needs replacement then do an entire drive shaft upgrade. It worth it, it's great truck.