wakaranaikoto ga attara, enryosinaide situmonsite kudasai.

In this sentence, こと turns 分からない into the things you don’t understand.

• 分からない = don’t understand / unclear
• 分からないこと = things that are unclear / anything you don’t understand

So 分からないことがあったら means something like:

• if there is anything you don’t understand
• if you have any questions
• if anything is unclear

Here, こと is a very common way to refer to an abstract matter, issue, or thing.

が marks 分からないこと as the thing that exists/comes up with ある.

The structure is:

• 分からないこと = something you don’t understand
• がある = exists / there is

So:

• 分からないことがある = there is something you don’t understand

Then あったら makes it conditional:

• 分からないことがあったら = if there is anything you don’t understand

Using は here would change the nuance and would not be the normal phrasing in this sentence.

あったら is the -たら conditional form of ある.

• ある = to exist, to have
• あったら = if there is / if there happens to be

So the first part literally works like:

• If there is a thing you don’t understand...

This is a very natural Japanese way to say:

• If you have any questions...
• If anything is unclear...

Japanese often expresses ideas through existence rather than in the exact same way English does.

Compare:

• 分からないことがあったら
  literally: if there is something you don’t understand

This sounds very natural in Japanese. English often prefers:

• if you don’t understand something
• if you have any questions

So this is a good example of how Japanese and English package the same meaning differently.

遠慮する means to hold back, to hesitate out of politeness, or to refrain.

So:

• 遠慮しないで = don’t hold back
• 遠慮しないで = don’t hesitate
• 遠慮しないで = please feel free to...

In this sentence, it softens the invitation and makes it sound welcoming:

• Please don’t hesitate to ask questions.
• Feel free to ask questions.

This phrase is very common in polite Japanese.

〜ないで is commonly used to mean without doing... or don’t do..., and it often links naturally to the next action.

Here:

• 遠慮しないで 質問してください
• literally: without hesitating, please ask questions

This is the standard phrasing.

遠慮しなくて can appear in some contexts, but in this sentence 遠慮しないで is the natural choice.

Both can exist, but 質問してください is more natural and common here.

• 質問する = to ask a question / to question
• 質問してください = please ask questions

You may also hear:

• 質問をしてください

But with many suru nouns, Japanese often drops を and just uses:

• 勉強する
• 電話する
• 質問する

So 質問してください sounds smooth and standard.

It is a polite request, not a harsh command.

• ください = please do ...

So 質問してください means:

• please ask
• please ask questions

Because the sentence also includes 遠慮しないで, the overall tone is friendly and encouraging, not forceful.

Japanese often leaves the subject unstated when it is obvious from context.

In this sentence, the listener is understood:

• (あなたが) 分からないことがあったら、(あなたは) 遠慮しないで質問してください。

But Japanese normally does not say あなた unless there is a reason to emphasize it.

So even though you is not written, it is clearly implied.

Both 分からない and わからない are common.

• 分かる is often written with the kanji 分
• but many native materials also write it in hiragana: わかる, わからない

Japanese writers often choose kana for readability or style. So:

• 分からないこと
• わからないこと

both mean the same thing.

Yes, depending on nuance.

• 質問してください = please ask questions
• 聞いてください = please ask / please ask me / please say it if you want to know

But 聞く is broader and can mean to ask or to listen/hear depending on context.
質問してください is more specific: it clearly means ask questions.

That makes 質問してください especially suitable in a classroom, meeting, or instruction setting.

It is polite, friendly, and encouraging.

The sentence does not just say ask questions. It says:

• If there’s anything you don’t understand, please don’t hesitate to ask questions.

That 遠慮しないで part is important because it reassures the listener and makes the speaker sound considerate.

This kind of sentence is very common from:

• teachers
• coworkers
• presenters
• customer support staff

Yes. That is a very natural English translation.

Japanese often says both parts:

• 分からないことがあったら = if there is anything you don’t understand
• 質問してください = ask questions

English often compresses this into:

• If you have any questions, please feel free to ask.

So even if the Japanese structure is more explicit, that English version matches the natural meaning well.