muzukasii bunpou ha sensei ni mou itido setumeisite moraimasita.

Using は makes 難しい文法 the topic of the sentence: as for the difficult grammar...

So the sentence is not just saying I had the teacher explain difficult grammar in a neutral way. It is more like:

• As for the difficult grammar, I had the teacher explain it to me one more time.

If you used を instead, the grammar would be treated more directly as the object:

• 難しい文法を先生にもう一度説明してもらいました。

That is also natural, but は gives a slight contrast or topical feel, as if this difficult grammar is what the speaker wants to talk about.

Because 難しい is an i-adjective, and i-adjectives directly modify nouns.

So:

• 難しい文法 = difficult grammar

You do not say:

• 難しいの文法 ❌

A similar pattern is:

• 新しい本 = a new book
• 面白い映画 = an interesting movie

Here, 先生に marks the person who performs the action for the speaker.

In the pattern:

• Vて + もらう

the person who does the favor is often marked with に.

So:

• 先生に説明してもらいました = I had the teacher explain it to me = The teacher explained it for me / as a favor to me

The important point is that に here does not mean location. It marks the giver of the action/favor.

This is the pattern:

• 説明して = te-form of 説明する (to explain)
• もらいました = past polite form of もらう

When もらう follows a te-form, it means to receive the favor of someone doing something.

So:

• 説明してもらいました = received the favor of having it explained = more naturally, had someone explain it to me

This is a very common and important structure in Japanese:

• 友だちに手伝ってもらいました。 = I had my friend help me.
• 母に料理を作ってもらいました。 = I had my mother make food for me.

The subject is omitted, but in most contexts it is understood as I.

So the full idea is something like:

• I had the teacher explain the difficult grammar to me one more time.

Japanese often leaves out subjects when they are clear from context. This is extremely common.

So even though there is no written 私 here, the listener usually understands the speaker is talking about themself.

They are related, but the viewpoint is different.

• 先生が説明してくれました。 = The teacher explained it for me.

  • Focus: what the teacher did for me

• 先生に説明してもらいました。 = I had the teacher explain it to me.

  • Focus: I received that favor

Both involve a benefit to the speaker, but:

• くれる centers on the giver
• もらう centers on the receiver

In this sentence, the speaker is presenting the event from their own side: I received the explanation from the teacher.

もう一度 means one more time or again.

It tells you that the teacher explained it again, not for the first time.

So:

• 先生にもう一度説明してもらいました = I had the teacher explain it to me one more time

It comes before 説明して because it modifies the action of explaining.

You will often see:

• もう一度言ってください。 = Please say it one more time.
• もう一度見せてください。 = Please show me again.

Because Japanese is basically an SOV language, and the main verb normally comes at the end.

A rough structure here is:

• 難しい文法は = as for the difficult grammar
• 先生に = from/by the teacher
• もう一度 = one more time
• 説明してもらいました = had it explained to me

Japanese often puts the main action last, so learners have to wait until the end to get the full grammatical meaning.

Yes.

• dictionary form: もらう
• polite present/future: もらいます
• polite past: もらいました

So 説明してもらいました means the receiving of the favor happened in the past.

That is why the whole sentence is in the past:

• I had the teacher explain it to me one more time.

Because the grammar pattern is:

• Vて + もらう

The te-form connects the main action to もらう.

So:

• 説明する → 説明して
• 説明してもらう

This does not mean explain and receive as two separate actions. Together they form one grammar pattern meaning:

• to receive the favor of someone explaining

This same pattern works with many verbs:

• 教えてもらう = to have someone teach you
• 手伝ってもらう = to have someone help you
• 書いてもらう = to have someone write it for you

Yes, in English we often want an object like it, but in Japanese it is often omitted when clear from context.

Here, 難しい文法は is the topic, and it gives us the thing being explained. So even though there is no explicit それを, the meaning is understood:

• As for the difficult grammar, I had the teacher explain it to me one more time.

Japanese frequently leaves out things that English would keep.

Yes, very often that is the nuance.

Vて もらう often suggests that the speaker arranged for, asked for, or benefited from someone doing something.

So this sentence can imply something like:

• the speaker needed help
• the teacher explained it again
• the speaker received that helpful action

It does not necessarily mean the speaker strongly forced the teacher to do it. It just presents the explanation as a beneficial action received by the speaker.

Yes, that is also correct and natural.

The difference is mainly in focus:

• 難しい文法は...

  • As for the difficult grammar...
  • topical, possibly contrastive

• 難しい文法を...

  • more neutral object marking
  • straightforward description of what was explained

So both work, but は gives a slightly stronger sense that the difficult grammar is the topic under discussion.

You could think of it that way as a learning tool, but it is not natural English.

A literal breakdown is something like:

• difficult grammar — topic
• from the teacher
• one more time
• received the favor of explaining

But the natural English translation is:

• I had the teacher explain the difficult grammar to me one more time.
• or The teacher explained the difficult grammar to me again.

When learning Japanese, a slightly literal gloss can help you understand the grammar, but natural translation should sound normal in English.