What is the difference between realisieren, merken and bemerken?
Als ich im Park spazierenging, realisierte/merkte/bermerkte ich, dass die Sonne unterging.
Actually, you can use all of them. You use realisieren when you suddenly notice someting although it was there before. merken/bemerken is more like watch something happening. One thing to edit: if you use als, realizing something appears to be a consequence. In this case, you may better use während (during).