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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.
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2018-09-28T11:27:01.343
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2018-09-29T04:57:29.653
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question
grammar giving-and-receiving
Changing verb subject after a favor - Is this a reliable pattern?
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.
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2018-09-28T16:24:57.713
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2018-09-29T04:57:29.653
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answer
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.
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answer