situmon ga areba, kigaru ni sensei ni kiite kudasai.

あれば is the conditional form of ある, which means to exist / to have.

So:

• 質問がある = there is a question / you have a question
• 質問があれば = if you have a question / if there are any questions

In this sentence, it sets up a condition for the second part:

• 質問があれば、気軽に先生に聞いてください。
• If you have any questions, please feel free to ask the teacher.

Here, が marks 質問 as the thing that exists with ある.

• 質問がある = there is a question / there are questions
• not 質問をある, because ある does not normally take を
• not 質問はある here, because that would shift the focus or contrast

Using が is the standard pattern with ある when you say that something exists or is present.

気軽 is basically a na-adjective / noun-like word meaning casual, relaxed, or without hesitation.
When it is used adverbially to describe how an action is done, it takes に.

So:

• 気軽な人 = an easygoing person
• 気軽に聞く = ask casually / ask without hesitation / feel free to ask

In this sentence, 気軽に describes how you should ask the teacher.

Not exactly. In English, casually can sound a little too informal.
Here, 気軽に has the feeling of:

• don’t hesitate
• feel free
• ask without worrying too much
• comfortably / openly

So the sentence is not telling you to be rude or overly casual with the teacher. It is encouraging you not to hold back if you need help.

に marks the target of 聞く here—the person you ask.

• 先生に聞く = ask the teacher
• literally, something like ask to the teacher

This is a very common pattern:

• 友だちに聞く = ask a friend
• 母に聞く = ask my mother

So in this sentence, 先生に means to the teacher / from the teacher as the person you direct the question to.

聞く can mean several related things:

• to hear
• to listen
• to ask

You tell from the context and the sentence pattern.

Here, because you have:

• 先生に
• and the topic is 質問 (questions)

it clearly means to ask.

So:

• 音楽を聞く = listen to music
• 話を聞く = listen to what someone says
• 先生に聞く = ask the teacher

聞いてください is a polite way to say please ask.

It is made from:

• 聞く → 聞いて (the te-form)
• ください = please

Together:

• 聞いてください = please ask

This form is commonly used for polite requests or instructions.

Compare:

• 聞く = dictionary form, plain
• 聞きます = polite statement, I ask / will ask
• 聞いてください = please ask

It could be either. Japanese often leaves that unspecified.

• 聞いてください can be said to one person
• It can also be said to a group

In English, you might translate it as:

• If you have any questions, please feel free to ask the teacher.
• or If any of you have questions, please feel free to ask the teacher.

The Japanese itself does not force you to choose.

Yes. Japanese often omits subjects when they are understood from context.

This sentence does not explicitly say:

• you
• students
• everyone

But the listener understands that the speaker means something like:

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

So the implied subject is probably you or anyone listening.

The comma separates the if-clause from the main request.

• 質問があれば、 = if you have any questions,
• 気軽に先生に聞いてください。 = please feel free to ask the teacher.

Japanese commas are often used more flexibly than English commas, but here it helps show the pause between the condition and the main statement.

Yes, it could, and the meaning would be very similar:

• 質問があれば = if you have any questions
• 質問があったら = if you have any questions / when you have questions

The difference is mainly in nuance and style:

• あれば sounds a bit more neutral or formal
• あったら sounds a bit more conversational

In school or polite instructional language, あれば is very natural.

Japanese does not use articles like a or the.
It also often omits words like your when the meaning is obvious.

So:

• 先生に聞いてください naturally means ask the teacher
• depending on context, it may really mean ask your teacher or ask the teacher in charge

English needs extra words, but Japanese often leaves them unstated.

Yes, it is polite and fairly standard.

What makes it polite:

• ください in 聞いてください
• the overall classroom/instructional tone
• あれば also sounds a little more polished than a very casual alternative

It is not extremely formal or stiff, but it is appropriate for a teacher, school notice, class instructions, or polite guidance.