As I know, when the potential form of a verb is being used, direct object particle "を" should not be used. Instead, が or は should be used. So when should we use which? For example,
富士山が 登れた climb Fuji Mountain.
重い荷物は持てます hold heavy baggage.
I don't see why in the second example, the particle は is used, because logically, "baggage" is the object of the verb "hold". I think it should be the same as in the first sentence, where が is used.
I'm a newbie learner. I would really appreciate it if you could help me with this question. Thanks!