«I am afraid your examples are insufficient. Simultaneity is not the only reason to use Simple Past in two or more actions.»
I wanted to find an example resembling yours, that is, meeteng the form
"someone doubted if + past simple" to express a doubt about an action taking place in the future by the moment of the doubt. That could prove your example is correct. But I found no such examples.
Of course, there were many sentences satisfying the abovementioned form. But in thouse the doubt and the action were simultaneous, in contrast to your example.
On the other hand, I found many sentences of the form:
"someone doubted if + [subject] would V1"
And it were theese examples which described a doubt preceding to the action in question.
From the written above I concluded that your example was either incorrect or very rare and asked a native speaker to tell his/her opinion.
«When actions are in sequence, it is a more common reason to use Simple Past:
Ex: He stopped there, stayed a while, and then went on along the road. (1) »
This is true. But this applies to actions not subordinated one to another.
«Therefore, my example is possible:
Ex: He doubted if they visited their uncle the next day. (2)»
The first and the second examples use different hypotaxis. The second example has a subordinate clause, while the first doesn't. The second sentence doesn't express just a sequence of past actions. The actions are subordinated in it. So, from the correctness of the first example doesn't follow the correctness of the second.
«== Visited happens after Doubted. As I have reasoned, now I (the speaker) know it is a fact, so I skip modal auxiliary.»
Let's suppose you are right. The sentece itself expresses that at a certain moment in the past they doubted about their visit on the next day. It says nothing about the visit itself, it doesn't say whether the visit took place or not.
On the other hand the use of simple past or of <would + V1>, as you assert, depends on whether you know the fact or not. If you didn't know if they had visited their uncle, you would use <would + V1>. This is nonsense because the information that this sentence should express is the same irregardless of whether you know if the visit took place or not: the information is about a doubt of an action, not about what happend later. This is another argument against the corectness of your example.
-----------
«My reply: There is no "future in the past". It is a past doubt from someone other than the writer or speaker himself:
Ex: He doubted they would go to visit their uncle.
In my book I have explained there is a past doubt, a doubt from an actor in the storybook. As for the writer's doubt, it is a present doubt to the past, using perfective.»
Future in the Past is very simple: three days ago yesterday was in the future. And I would say: "I will finish my work on the day after tomorrow". But today it would sound like this:
"Three days ago I said I would finish my work in two days." That is Future in the Past. It refers to any moment or period that was in the future at a certain moment in the past.
Analogously, <will have done> may be called Past in the Future.
«Calling a tense "future in the past" proves one doesn't know how to define the future. Teaching a student to look for a future in the past will turn his common sense upside down.»
I find the name quite logical, characterising what this tense is used for.
You asked: "Do tenses denote time?"
Not 'Yes', and not 'No'.
If we want to determine the time of an action described in a sentence we should look at the tense and at the context. In general case only the both can clearly determine the time.
I wanted to find an example resembling yours, that is, meeteng the form
"someone doubted if + past simple" to express a doubt about an action taking place in the future by the moment of the doubt. That could prove your example is correct. But I found no such examples.
Of course, there were many sentences satisfying the abovementioned form. But in thouse the doubt and the action were simultaneous, in contrast to your example.
On the other hand, I found many sentences of the form:
"someone doubted if + [subject] would V1"
And it were theese examples which described a doubt preceding to the action in question.
From the written above I concluded that your example was either incorrect or very rare and asked a native speaker to tell his/her opinion.
«When actions are in sequence, it is a more common reason to use Simple Past:
Ex: He stopped there, stayed a while, and then went on along the road. (1) »
This is true. But this applies to actions not subordinated one to another.
«Therefore, my example is possible:
Ex: He doubted if they visited their uncle the next day. (2)»
The first and the second examples use different hypotaxis. The second example has a subordinate clause, while the first doesn't. The second sentence doesn't express just a sequence of past actions. The actions are subordinated in it. So, from the correctness of the first example doesn't follow the correctness of the second.
«== Visited happens after Doubted. As I have reasoned, now I (the speaker) know it is a fact, so I skip modal auxiliary.»
Let's suppose you are right. The sentece itself expresses that at a certain moment in the past they doubted about their visit on the next day. It says nothing about the visit itself, it doesn't say whether the visit took place or not.
On the other hand the use of simple past or of <would + V1>, as you assert, depends on whether you know the fact or not. If you didn't know if they had visited their uncle, you would use <would + V1>. This is nonsense because the information that this sentence should express is the same irregardless of whether you know if the visit took place or not: the information is about a doubt of an action, not about what happend later. This is another argument against the corectness of your example.
-----------
«My reply: There is no "future in the past". It is a past doubt from someone other than the writer or speaker himself:
Ex: He doubted they would go to visit their uncle.
In my book I have explained there is a past doubt, a doubt from an actor in the storybook. As for the writer's doubt, it is a present doubt to the past, using perfective.»
Future in the Past is very simple: three days ago yesterday was in the future. And I would say: "I will finish my work on the day after tomorrow". But today it would sound like this:
"Three days ago I said I would finish my work in two days." That is Future in the Past. It refers to any moment or period that was in the future at a certain moment in the past.
Analogously, <will have done> may be called Past in the Future.
«Calling a tense "future in the past" proves one doesn't know how to define the future. Teaching a student to look for a future in the past will turn his common sense upside down.»
I find the name quite logical, characterising what this tense is used for.
You asked: "Do tenses denote time?"
Not 'Yes', and not 'No'.
If we want to determine the time of an action described in a sentence we should look at the tense and at the context. In general case only the both can clearly determine the time.