sensei ni nihongo de situmonsitara, sono ba de wakariyasui setumei wo site kuremasita.

先生に marks the person the question was directed to.

In this sentence:

• 先生に質問する = to ask the teacher a question

This is different from:

• 先生が = the teacher is the subject
• 先生を = would make the teacher the direct object, which does not fit 質問する here

So に shows the target/recipient of the asking.

日本語で means in Japanese.

The particle で is used here to show the means/method or language used to do something.

So:

• 日本語で質問する = ask a question in Japanese
• 英語で話す = speak in English
• 箸で食べる = eat with chopsticks

Here, で tells you the language used for the question.

質問したら is the -tara form of 質問した.

In this sentence, ～たら means something like:

• when
• after
• if

So 先生に日本語で質問したら means:

• when I asked the teacher in Japanese
• or after I asked the teacher in Japanese

Here it describes a sequence: first the speaker asked, then the teacher explained.

A key nuance is that ～たら often presents the second part as something that happened as a result or immediately afterward.

In this sentence, it is most naturally understood as when or after, not a hypothetical if.

Why? Because the second part is a real completed event:

• してくれました = did for me

So the sentence is talking about something that actually happened:

• I asked the teacher in Japanese, and then the teacher explained it clearly on the spot.

If it were truly hypothetical, the rest of the context would usually make that clearer.

その場で means on the spot, right there, or then and there.

Breakdown:

• その = that
• 場 = place, scene, situation
• で = at/in

So その場で does not just mean a physical place. It often means immediately in that situation, without delay.

Examples of the nuance:

• その場で答える = answer on the spot
• その場で決める = decide right then and there

It is the same particle で, but the role is a little different.

• 日本語で: で marks the means/language
• その場で: で marks the place/situation where the action happens

So で has multiple common uses, and context tells you which one it is.

分かりやすい means easy to understand.

It is built from:

• 分かる = to understand / to be understandable
• やすい = easy to do

So literally it is something like:

• easy to understand

This pattern is very common:

• 読みやすい = easy to read
• 話しやすい = easy to speak / easy to talk with
• 使いやすい = easy to use

Even though 分かる is not usually translated as to understand in the same way as English grammar works, 分かりやすい is a fixed and very common expression meaning clear or easy to understand.

Both are possible.

• 説明をしてくれました
• 説明してくれました

The version with を makes the noun 説明 more explicit:

• 説明 = explanation
• 説明をする = to give an explanation / to explain

Japanese often uses noun + をする constructions, especially with Sino-Japanese nouns like:

• 勉強をする
• 質問をする
• 説明をする

So 説明をしてくれました is completely natural, though 説明してくれました is also common and slightly more compact.

～てくれました shows that someone did something for the speaker’s benefit.

Compare:

• 説明しました = explained
• 説明してくれました = explained it for me / kindly explained it to me

So くれる adds the nuance that the action was helpful, favorable, or appreciated by the speaker.

That is why this sentence feels more personal than a plain factual statement.

Yes, the subject is understood to be the teacher.

Japanese often leaves out subjects when they are clear from context. In this sentence:

• first clause: I asked the teacher
• second clause: the person who gave the explanation is naturally understood to be the teacher

So even though 先生は is not stated, it is understood.

Because Japanese adjectives come before the noun they modify.

So:

• 分かりやすい説明 = an easy-to-understand explanation

This is similar to:

• 面白い本 = an interesting book
• 長い話 = a long story
• 親切な先生 = a kind teacher

Since 分かりやすい is an i-adjective, it directly modifies 説明.

In this sentence, 日本語で most naturally attaches to 質問したら:

• 先生に 日本語で 質問したら
• when I asked the teacher in Japanese

That is the most straightforward reading because of the word order.

If you wanted to say the explanation was in Japanese, you would more naturally place it near 説明 or してくれました, for example:

• 先生に質問したら、日本語で分かりやすい説明をしてくれました
• 先生に質問したら、日本語で説明してくれました

So the position of 日本語で matters.

They are related, but not identical.

• 質問する = to ask a question
• 聞く = to ask / to hear / to listen

質問する is more specific and a bit more formal: it clearly means asking a question.

Examples:

• 先生に質問する = ask the teacher a question
• 先生に聞く = ask the teacher / ask the teacher about it

So in this sentence, 質問する emphasizes that the speaker asked an actual question, not just casually asked about something.

The sentence ends with してくれました, which is polite past.

That makes sense because:

• the speaker is talking about a completed event in the past
• 先生 is involved, so polite style is very natural
• it may be part of a polite conversation or written explanation

The plain form would be:

• 先生に日本語で質問したら、その場で分かりやすい説明をしてくれた。

The meaning is basically the same, but the tone is less formal.

Yes. 先生 is not the main topic of the whole sentence here. Instead, it is introduced as the person receiving the question:

• 先生に = to the teacher

Japanese often avoids adding extra topic or subject markers when the role is already clear.

If you said:

• 先生は日本語で質問したら…

that would sound wrong, because 先生は would make the teacher the topic in a way that does not fit the meaning.

If you said:

• 先生がその場で分かりやすい説明をしてくれました

that would explicitly mark the teacher as the subject of the second part, which is possible in some contexts, but not necessary here.

It breaks down like this:

• 先生に = to the teacher
• 日本語で = in Japanese
• 質問したら = when/after I asked
• その場で = on the spot
• 分かりやすい説明を = an easy-to-understand explanation
• してくれました = gave/explained for me

So the pattern is:

[When I did X], [someone did Y for me].

More literally:

When I asked the teacher in Japanese, they gave me an easy-to-understand explanation right there.