Nuanced difference between the noun doubt and doubts

[xShun]

Here are the examples from Cambridge Online Dictionary

A: I'm having doubts about his ability to do the job.
B: I always had some doubt about the scheme but I never expressed it.

What is the nuanced or grammatical difference between sentence A and B?

The following are my surmises. I’m not sure my understanding is correct or not.
About the plural form “doubts”, from sentence A, if I want to emphasise the “specific” things I cast doubt/suspicion on, I shoould use the plural form doubts. And I have to specify what kind of things I doubt afterwards, I guess?

About the single form “doubt” from sentence B, I have different understanding and I don’t know which one is correct.
From sentence B, here the "doubt" is being emphasised about the “feeling of being uncertain about something or not believing something”. And since “feeling” cannot be counted, so we use “doubt”.

But my Japanese teacher teaching English told me that: From sentence B, here the “doubt” is functioned as a verbal noun. It is an action, so it is uncountable and we use “doubt”. But somehow I don't buy it. Is he right?

Furthermore, depending on the intention that a speaker wants to emphasise, can we swap the usage between “doubt” and ”doubts”? Like this:

C: I'm having doubt about his ability to do the job.
D: I always had some doubts about the scheme but I never expressed it.

 

[probus]

To me there is no practical difference in meaning. Doubt is one of those nouns that can be countable and non-countable with no difference.

 

[emsr2d2]

I wouldn't use sentence C.

 

[jutfrank]

About the plural form “doubts”, from sentence A, if I want to emphasise the “specific” things I cast doubt/suspicion on, I shoould use the plural form doubts. And I have to specify what kind of things I doubt afterwards, I guess?

It means that you are starting to lose confidence in his ability. The plural form suggests that the doubting has occurred on multiple occasions.

About the single form “doubt” from sentence B, I have different understanding and I don’t know which one is correct.
From sentence B, here the "doubt" is being emphasised about the “feeling of being uncertain about something or not believing something”. And since “feeling” cannot be counted, so we use “doubt”.

Yes, that's a reasonable understanding. You can paraphrase this to mean 'a feeling of being unsure' about the scheme. That means the speaker is not fully convinced the scheme is a good idea, at least in its current form.

But my Japanese teacher teaching English told me that: From sentence B, here the “doubt” is functioned as a verbal noun. It is an action, so it is uncountable and we use “doubt”. But somehow I don't buy it. Is he right?

No, doubting is not an action.