Roman generals improve on Greek military tactics by employing?

This isn't a question, it's an incomplete sentence... That said, it is hard to say that Roman tactics are explicitly better than Greek ones, just are different. They were both infantry heavy armies that relied on orderly ranks of troops, but there were significant changes made by the Romans.

The Greeks relied heavily on the phalanx formation including the hoplites. These units were slow moving, hard hitting powerhouses that emphasized lots of soldiers in a small area, meaning they could outnumber troops on the front line who spread themselves out more thinly.

The Romans did away with the phalanx formation, allowing their infantry to move more quickly than more traditional Hellenic formations. At the same time, Roman units were more concentrated than many northern armies which often did not keep their troops in orderly ranks, giving them the same advantage over them the Greeks had. They were able to maintain their fighting capabilities when crunched into these tight formations by using the Gladius Hispaniesis, a sWord which was thrust at the enemy instead of swung, allowing Roman soldiers to fight shoulder to shoulder. Roman foot soldiers also carried a few Pila, or javelins to throw into the enemy ranks before charging in order to injure and demoralize. All of this adds up to a mobile, but still well protected and powerful military unit, well trained and able to execute complex tactical maneuvers at the general's command.