How does Sentinel Prime have an earth based ALT-mode even when he had crashed on the moon ? The Autobots find him in the exact same state. So my question is how could he have an earth based ALT-mode even before he had ever visited earth?
Since you mention the moon, I assume you're referring to Sentinel Prime in the film "Transformers 3: Dark Side of the Moon" (there have been numerous characters named Sentinel Prime throughout the various continuities).
Spoilers, just in case