Consider the following example sentences from 新完全文法マスター N1 and N2 respectively:
In both of these examples, there is the same pattern:
- I/You ask a person for a favor.
- A conditional conjugation (ば・たら)
- The person does the favor to me/you.
As a concrete example, let's map the pattern to the sentences:
You ask Kawada for a favor. (Subject = You)
A conditional conjugation (ば)
- Kawada does the favor to you. (Subject = Kawada)
I ask the old woman for a favor. (Subject = Me)
A conditional conjugation (たら)
- The old woman does the favor to me. (Subject = Old woman)
If we consider the composition of sentences like these, the following question arises: Should the second part of the sentence have the same subject as the first part, or not?
The sentences might be possible to write without changing the verb subject if もらう was used instead of くれる in the second part. However, in both of these examples, くれる was used, and the subject changed between the two parts.
My question is: Is the 3-step pattern mentioned above a useful indicator that the sentence will change verb subject (and therefore should use くれる rather than もらう)?
Alternatively, is there another explanation for why the subject should change, or why くれる is chosen instead of もらう in these cases?
At minimum, it seems like the 3-step pattern is not bulletproof. For example, "A Dictionary of Basic Japanese Grammar" has the following example sentence in it:
You'll understand it if you ask Professor Matsumoto.
In this counter-example, the subject of both verbs in the sentence is "you", since "you" understand after "you" ask.