How to Troubleshoot a Standard Transmission

1). Start the vehicle and attempt to put the vehicle into first gear. If you experience any crunching noises while you do this (or when you shift into any gear while driving), it means that the clutch is not fully disengaged and you are grinding the teeth on the transmission gears. This is obviously not good. The clutch must be adjusted by a professional mechanic.
2). Check the transmission fluid. This is one of the more common causes for what seems to be a failing or malfunctioning transmission. Pull the dipstick out of the transmission filler neck and wipe it off with a shop rag. Put the dipstick back into the filler neck all the way until it bottoms out, then remove it again. Check the oil level. It should be between the upper and lower marks on the end of the stick. If it is not, add more fluid to the transmission using a funnel and fresh transmission oil.
3). Check to see if your vehicle slips out of gear while you are driving it. If you find it difficult to stay in gear while driving, it's probably because the clutch disc is slipping off the flywheel and so your engine and transmission are intermittently becoming disconnected. To fix this, have your clutch disc and assembly replaced and your flywheel resurfaced.