A question about the particle used with potential form


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.




> 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.

That's not really correct; you can still use を naturally in many cases. See:
[The difference between が and を with the potential form of a
verb.](https://japanese.stackexchange.com/q/609/5010)

> が or は should be used. So when should we use which?

So do you know [the standard difference between が and
は](https://japanese.stackexchange.com/q/22/5010)? Can you explain the
difference between [先生は来る and
先生が来る](https://japanese.stackexchange.com/q/43213/5010), for example? If yes,
there is nothing special here. Please keep in mind that [Japanese potential
forms are similar to English
"-able"](https://japanese.stackexchange.com/a/36482/5010). は is used to
topicalize a word or making a contrast, whereas が is used for "neutral
description" or "exhaustive listing".

>   * 重い荷物が持てます。 (neutral)  
>  I can carry heavy baggage.
>   * 重い荷物は持てます。 (contrastive _wa_ )  
>  I can (at least) carry heavy baggage (although I cannot do something else).
>

Another example:

> **A** : この3人の中で、誰 **が** 英語を話せますか？ Who can speak English among the three?  
>  **B** : 太郎 **が** 話せます。 Taro can speak English.  
>  (exhaustive-listing _ga_ ; implies he is the only person who can speak
> English among the three)  
>  **C** : 花子は中国語 **は** 話せますよ。 (But) Hanako can speak Chinese!  
>  (contrastive _wa_ ; implies she cannot speak English)




> 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.

That's not really correct; you can still use を naturally in many cases. See:
[The difference between が and を with the potential form of a
verb.](https://japanese.stackexchange.com/q/609/5010)

> が or は should be used. So when should we use which?

So do you know [the standard difference between が and
は](https://japanese.stackexchange.com/q/22/5010)? Can you explain the
difference between [先生は来る and
先生が来る](https://japanese.stackexchange.com/q/43213/5010), for example? If yes,
there is nothing special here. Please keep in mind that [Japanese potential
forms are similar to English
"-able"](https://japanese.stackexchange.com/a/36482/5010). は is used to
topicalize a word or making a contrast, whereas が is used for "neutral
description" or "exhaustive listing".

>   * 重い荷物が持てます。 (neutral)  
>  I can carry heavy baggage.
>   * 重い荷物は持てます。 (contrastive _wa_ )  
>  I can (at least) carry heavy baggage (although I cannot do something else).
>

Another example:

> **A** : この3人の中で、誰 **が** 英語を話せますか？ Who can speak English among the three?  
>  **B** : 太郎 **が** 話せます。 Taro can speak English.  
>  (exhaustive-listing _ga_ ; implies he is the only person who can speak
> English among the three)  
>  **C** : 花子は中国語 **は** 話せますよ。 (But) Hanako can speak Chinese!  
>  (contrastive _wa_ ; implies she cannot speak English)

