How exactly do time and "tense" work in Mandarin REALLY?
If the form 我看了一本书 is correct then what role is 了 playing here and why does it sound to me like past tense with a not-a-tense-marker indicating tense?

'了' in &quot;我看了一本书&quot; is a [aspect marker] indicating completed action.

我看一本书 = I read a book

In 我看了一本书,  the action '看' is completed, which imply 看 is not in present tense but in past tense.


Also, depend on the context, [verb + 了] could indicate :

[present perfect tense] 我刚看了一本书 = I have just read a book
[past perfect tense] 我昨天看了一本书 = I had read a book yesterday
[future perfect tense] 我明天会看了这本书 = I will have read this book tomorrow.

'过' in &quot;我看过一本书&quot; is a [particle] marking experiential aspect

我看一本书 = I read a book
我看过一本书 = I had read a book (I experienced reading a book, that indicates 看 is in past perfect tense)

*I think I answered a similar question How do you specify past tense for 是?

If no time reference is found in the context, then all verbs are in present tense by default.
When you are reading a historical novel, even most individual sentences in it didn't have time reference, you should still know it is in past tense; If you are reading an instructional manual, you know the text is written in present tense entirely.

Edit:

Mike3 comment:
OK... now I'm not entirely clear on the difference in meaning between &quot;我看了一本书&quot; and &quot;我看过一本书&quot;. Is it just that one emphasizes something else (completion vs. experience) or what? Which would be the correct one to use in the scenario I mentioned in the question and why?

In your scenario:
Q: &quot;How Do you know?&quot;
A: &quot;I read a book&quot;
you can use &quot;我看了一本书&quot; or &quot;我看过一本书&quot; for &quot;I read a book&quot;  in this scenario.

&quot;我看了一本书&quot; stated the fact that you finished reading a book in the past. (了 emphasizes the completeness of your action)
&quot;我看过一本书&quot; stated the fact that you had in the past read a book (过 emphasizes the action was in the past).

Both sentences stated the source of your knowledge is from reading a book. '看过' is more preferable if you want to emphasize the action happened in the past.
As I mentioned, depend on the context, '看了' can be in any perfect tense.