Changing verb subject after a favor - Is this a reliable pattern?

Question

Consider the following example sentences from 新完全文法マスター N1 and N2 respectively:

Sentence A

その仕事は川田さんならやれる。川田さんに頼めばきっとやってくれるだろう。

Sentence B

道に迷ってしまった。通りかかったおばあさんに道を聞いたら、親切に教えてくれた。

In both of these examples, there is the same pattern:

1. I/You ask a person for a favor.
2. A conditional conjugation (ば・たら)
3. The person does the favor to me/you.

As a concrete example, let's map the pattern to the sentences:

Sentence A

川田さんに頼めばきっとやってくれるだろう。

...

川田さんに頼めば

• You ask Kawada for a favor. (Subject = You)

• A conditional conjugation (ば)

きっとやってくれるだろう

• Kawada does the favor to you. (Subject = Kawada)

Sentence B

通りかかったおばあさんに道を聞いたら、親切に教えてくれた。

...

通りかかったおばあさんに道を聞いたら、

• I ask the old woman for a favor. (Subject = Me)

• A conditional conjugation (たら)

親切に教えてくれた。

• 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:

これは松本先生に聞けば分かります。

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.

Answer 1

Inspired by user4092's comment, I did more research, and I think I understood what's going on here. I'll write my theory for others to judge.

It's important to understand one difference between くれる and もらう:

• くれる is like "somebody does something".

• もらう is like "have somebody do something".

Let's apply this interpretation of くれる・もらう to the example sentences:

Sentence A

川田さんに頼めばきっとやってくれるだろう。

If (you) entrust Kawada, he will certainly do it for you.

Sentence B

おばあさんに道を聞いたら、親切に教えてくれた。

After asking the old woman, she kindly taught me.

Viewed from these translations, the sentences make perfect sense.

Now, let's see what happens if we avoid changing the subject by using もらう instead.

Sentence A

× 川田さんに頼めばきっとやってもらうだろう。

× If (you) entrust Kawada, you will have him certainly do it.

Sentence B

× おばあさんに道を聞いたら、親切に教えてもらった。

× After asking the old woman, I had her kindly teach me.

Even if we ignore the awkwardness of quasi-literal translation, isn't it clear that there's something weird about this phrasing?

• If you've already entrusted Kawada, why "have him" do it? You already entrusted him!
• If you've already asked the old woman, why "have her" teach you? You already asked her!

Based on this interpretation, the awkwardness of もらう is clear: It sounds redundant.

Now, to answer my original questions:

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?

Yes, seen from the perspective of くれる and もらう, this type of sentence will likely change subject. The very purpose of this sentence structure is to say what somebody does after you ask them to do something, so the subject naturally changes. If you don't change the subject by using もらう, then you end up with the redundant-sounding sentences shown above.

Maybe this answer will look like over-thinking to others... but for some reason, this was hard for me to wrap my head around. I hope this can help somebody else avoid the same confusion.

Answer 2

If you use もらう, you have to conjugate it into もらえる and get rid of volitionality. So, using くれる can make it simpler. (If the sentence B was not 道を聞いたら but 道を聞いて, you could use もらった as it is, though it would change the nuance a bit.)

Moreover, sentence A is more consistent with Kawata as the subject because it follows a sentence with Kawata as the subject as well, apart from the sub clause.

Edit: As for your additional question, when Aたら Bした pattern is used for simple temporal sequence (not imaginary conditional or habitual past), you can't use an action that's under control of your own for B. In this regard, もらえた is fine because being able to do something is not done at your will. 道を聞いて教えてもらった is "I asked someone about the way and had him/her tell me".

川田さんに頼めば教えてもらう means that entrusting kawata is a trigger to motivate you to get information. On the other hand, …頼めば教えてもらえる means that it's a trigger to enables you to get information.