Why won't my Opel Corsa start I have replaced some parts but still it won't start?

As in other answers it could be many things, the following is the checks I make when facing this problem.

1: Is there fuel?

2: Is there a spark? Remove number 1 plug, mark and remove the other leads, hold the threaded part of number 1 plug against the engine with insulated pliers, ask someone to turn the engine over. You should have a nice blue spark, if the spark is there but weak, regap ALL the plugs.

If the engine doesn't fire at this point then I would check the ignition timing. Its not that difficult to do. With number 1 plug removed (number 1 plug BTW is the one nearest the crankshaft pulley end) AND ALL OTHER LEADS REMOVED, turn the engine over BY HAND using a socket on the crankshaft pulley, look for the timing marks on the pulley and the casing covering the pulley. Some-people find it easier if these markings are marked with "Tipex" or white paint. These will line up twice, once will be the compression stroke the other will be the exhaust stroke. You want the compression stroke. When the marks are ALMOST lined up put number 1 plug back into its lead (but not into the engine) and hold it against the engine again. Turn the ignition on BUT DO NOT TURN THE ENGINE OVER, continue to turn the engine over until you pas the timing marks and see if the plug sparks. It should spark about 5 degrees before top dead centre, if it doesn't spark continue to turn the engine by hand until the marks line up again (or just before) and he first time you turned the engine it may have been on the exhaust stroke! Hence the second rotation.

If the ignition timing is out (highly unlikely as most are controlled by the ECU nowadays) see a specialist.

The other thing that will stop the engine from starting is if the valve timing has slipped. The easiest way to check it (though not entirely accurate) is to remove the camshaft cover, look at the lobes (the egg shaped things, there are 8 of them) above number 1 piston. Rotate the engine by hand, as above until the marks are lined up, now the "pointy" parts of the lobes should be pointing as far away from the valve tops as possible. If you continue to turn the engine over you see them starting to turn towards the valve tops.

The valve timing will only change if the timing belt has slipped/snapped. Either way it is a garage job to repair it.