Learn Hardcore Japanese: riyuu wo kiitara, kare mo nattokusita. - When he heard the reason, he was convinced.

In this sentence, 聞いたら most naturally means “when/after (I) asked (or heard) the reason”—a completed action that triggered the next event.
The た in たら is the perfective (“completed”) form, not necessarily past in the English sense; it often reads as “once/when.”
With both clauses in past (聞いたら … 納得した), it describes a real sequence: I asked → he became convinced.
If you wanted a general conditional (“if you ask, he will…”), you’d use something like 聞けば or a non-past in the main clause: 理由を聞いたら、彼も納得する.

Who is doing the asking in 理由を聞いたら? It’s not stated.

Japanese often omits subjects. Here, context decides. The most likely reading is “When I/We asked (someone) the reason, he also became convinced.”
To make it explicit:
- 私が理由を聞いたら、彼も納得した。 When I asked the reason, he was convinced too.
- 彼が理由を聞いたら、私も納得した。 When he asked the reason, I was convinced too.

Does 聞く here mean “ask” or “hear/listen”?

聞く can mean both “to ask” and “to hear/listen,” and context decides.
With Xを聞く, it can be “ask about X” or “hear X.” So 理由を聞いたら can be “when (I) asked about the reason” or “when (I) heard the reason.”
If you want to unambiguously mean “ask (a person),” add the person with に: 先生に理由を聞いた (I asked the teacher the reason).
If you want a verb that only means “ask (a question),” you can use 尋ねる: 理由を尋ねた.

Why is it 理由を聞く and not 理由に聞く?

Pattern:
- Thing you ask about or hear: Xを聞く (e.g., 理由を聞く, 名前を聞く, 音楽を聞く).
- Person you ask: 人に聞く (e.g., 田中さんに理由を聞く).
So you can combine both: 田中さんに理由を聞いた = I asked Tanaka for the reason.

What does 彼も add? Why use も?

も means “also/even.” 彼も納得した = “He also (even he) was convinced.”
It implies that someone else (or others) had already been convinced, or that his being convinced is noteworthy.
Without も (彼は/彼が納得した), it’s just “He was convinced,” with no additive nuance.

What exactly does 納得する mean? Is it just “understand”?

納得する = to be convinced; to accept something after understanding the reasoning; to find it satisfactory.
It’s stronger than simple cognitive understanding (理解する) and different from acknowledgement (了解する) or humble acceptance (承知する).
Useful set phrase: 納得がいく/いかない = “(It) makes/doesn’t make sense to me; I can/can’t accept it.”

Is 納得した transitive? How do I say “I convinced him”?

納得する is intransitive from the convinced person’s perspective: 彼が納得した = He became convinced.
To say “I convinced him,” use a causative: 彼を納得させた.
Politer/softer: 彼に納得してもらった (I got him to be convinced).

Can I use other connectives instead of たら? How do they differ?

たら: “when/after,” a one-time trigger or discovery. Natural in narratives. Example: 理由を聞いたら、彼も納得した.
と: “when/whenever,” natural/automatic outcomes or discoveries; less about a deliberate action leading to persuasion here.
ば: more hypothetical/general: 理由を聞けば、彼も納得する (If you ask, he’ll be convinced).
から/ので: explicit cause/reason: 理由を聞いたから、彼も納得した (Because he heard the reason, he was convinced). ので is a bit softer/formal.
たところ: “upon doing/when I tried,” emphasizes the result upon attempting: 理由を聞いたところ、彼も納得した.

Could I connect with て instead: 理由を聞いて、彼も納得した?

Yes. て-form can mean “and/then,” often implying sequence or cause.
Nuance:
- 聞いて: looser connection; can read as “asked and (as a result) he was convinced.”
- 聞いたら: clearer “upon/when” trigger. Both are fine; たら highlights the timing/trigger more.

How do I make the sentence polite or suitable for business?

Polite plain: 理由を聞いたら、彼も納得しました。
More formal/polite with humble/honorific forms (e.g., for customers/clients):
- 理由をお伺いしたところ、ご納得いただきました。
- Avoid 納得されました for customers; prefer ご納得いただきました.

Is using 彼 natural? I’ve heard pronouns are avoided in Japanese.

彼 is fine in writing and explanations. In casual speech, people often use names, titles, or just omit the subject.
Also note 彼 can mean “boyfriend” in everyday talk, so speakers often prefer a name or その人 instead.
Alternatives: 田中さんも納得した。 / just （相手は）納得した。

How do I make it crystal clear who asked whom?

Add subject and the person asked:
- 私が彼に理由を聞いたら、彼も納得した。 When I asked him the reason, he was convinced too.
- 上司が部下に理由を聞いたら、部下も納得した。 When the boss asked the subordinate the reason, the subordinate was also convinced.
To mean “when I heard his reason,” say: 彼の理由を聞いたら、私も納得した。

Why is there no topic marker like は? What role is も playing?

も often stands in the slot of は/が while adding “also/even.” So 彼も納得した covers both the subject/topic and the “also” nuance.
You wouldn’t say 彼もは/彼もが here; も replaces them.

What happens if I move も to a different place, like 理由も聞いたら?

理由も聞いたら、彼は納得した。 = “When I also asked the reason (in addition to something else), he was convinced.” Here, も modifies 理由 instead of 彼.
Placement of も changes what is being treated as “also/even.”

How do you read the sentence?