why does my car die as soon as I put it in gear?

Possible causes are idling speed is too low, engine performance is poor or there is a fault with the torque converter.

It was running fine hours before. What would be the most likely a answer with that in mind?

What is the idling speed when it tends to stall? Are you able to restart easily?

It idles fine with or without giving it gas. As soon as I put it in gear, even giving it gas, it stalls.

Looks like you have a fault with the torque converter. This is an automatic transmission right?

No. It's a manual

That would be something else and this is rather surprising.

Check if the air hose between air filter and throttle body is cracked. You need to move it little with engine running in neutral to see if it affects anything.

Check for wires etc that have gotten into the way of the clutch pedal and related moving parts of the clutch.

Clutch cylinder?

Clutch cylinder won't be affecting this. Pedal movement and linkages