Had Julius Caesar lived longer, he would have consolidated his power as the first Emperor of Rome.
If Julius Caesar could have lived longer, he would have consolidated his power as the first Emperor of Rome.
Had Julius Caesar lived longer, he would have consolidated his power as the first Emperor of Rome.
If Julius Caesar could have lived longer, he would have consolidated his power as the first Emperor of Rome.
The structure "had Julius Caesar lived longer" is perfectly correct and is an appropriate way to introduce a contrary-to-fact hypothetical event. The structure "if Julius Caesar could have lived longer" is colloquial, longer, and not up to GMAT standards.