kono sensei nara, nihongo no benkyou ni tuite kigaru ni soudan dekimasu.

Here, この先生なら means something like:

• if it’s this teacher
• when it comes to this teacher
• as for this teacher, you can...

It does not just identify the teacher. It adds a nuance of judgment or selection: among possible teachers, this one is someone you can comfortably consult.

So the feeling is closer to:

• This teacher is the kind of teacher you can talk to
• If you’re talking about this teacher, it’s easy to ask for advice

なら often gives a sense of:

• if it’s X
• speaking of X
• as for X in particular

So:

• この先生は = simply this teacher...
• この先生なら = if it’s this teacher... / this teacher, at least...

Using なら suggests contrast or comparison, even if it is only implied. For example, it can suggest:

• maybe other teachers are harder to approach
• but this teacher is easy to talk to

So なら sounds more like a recommendation or reassuring comment.

That is a very natural question.

Yes, 相談する often appears as:

• 先生に相談する = to consult the teacher

But in this sentence, この先生なら is being used first as a topic/condition:
As for this teacher / If it’s this teacher...

That means the sentence is focusing on the teacher as the basis for the statement, rather than explicitly marking the teacher with に.

A more explicit version could be:

• この先生になら、日本語の勉強について気軽に相談できます。

This makes the teacher more clearly the person you consult to.

So:

• この先生なら... = natural, focuses on this teacher as a good option
• この先生になら... = also natural, makes the consult-to this teacher relationship clearer

日本語の勉強 means:

• the study of Japanese
• Japanese study
• studying Japanese

The の links the two nouns:

• 日本語 = Japanese language
• 勉強 = study

So 日本語の勉強 is a noun phrase meaning study of Japanese.

This is different from:

• 日本語を勉強する = to study Japanese

That is a verb phrase. In your sentence, a noun phrase is useful because it is followed by について.

Both are possible, but 日本語の勉強について is shorter and more natural here.

Compare:

• 日本語の勉強について = about studying Japanese
• 日本語を勉強することについて = about the act of studying Japanese

The second one is more explicit and heavier. Japanese often prefers the more compact noun phrase when possible.

So this sentence uses the smoother, more natural option.

について means:

• about
• regarding
• concerning

So:

• 日本語の勉強について相談する = to consult someone about studying Japanese

It marks the topic/content of the consultation.

A useful pattern is:

• N について話す = talk about N
• N について考える = think about N
• N について相談する = consult about N

気軽に means something like:

• casually
• freely
• without hesitation
• feel free to

In this sentence, it does not mean careless or unimportant. It means the teacher is approachable, and you can ask for advice without feeling stiff, nervous, or intimidated.

So the nuance is:

• You can feel free to talk to this teacher
• This teacher is easy to approach

気軽 is a na-adjective.

When a na-adjective modifies a noun, it uses な:

• 気軽な先生 = an approachable / easygoing teacher

When it modifies a verb, it usually uses に:

• 気軽に相談する = to consult casually / without hesitation

So here 気軽に is the adverb form, modifying 相談できます.

相談できます is the polite potential form of 相談する.

Breakdown:

• 相談する = to consult / to ask for advice
• 相談できる = can consult / can ask for advice
• 相談できます = polite form of 相談できる

So it means:

• can consult
• are able to ask for advice

In this sentence, the meaning is not physical ability. It is more about the situation being comfortable and appropriate:

• You can feel free to consult this teacher

The subject is omitted, which is very common in Japanese.

Depending on context, it could mean:

• you can consult
• one can consult
• we can consult
• students can consult

In many learning examples like this, the most natural interpretation is you or people in general.

So the sentence feels like:

• With this teacher, you can feel free to ask about studying Japanese

Yes. It is polite but not overly formal.

The politeness mainly comes from:

• できます instead of plain できる

So:

• 相談できます = polite
• 相談できる = plain

The overall tone is friendly and appropriate for normal conversation, school situations, or written guidance.
Also, 気軽に gives the sentence a warm, welcoming tone.

The comma is helpful, but not absolutely required every time.

It shows a natural pause after the topic/condition:

• この先生なら、日本語の勉強について気軽に相談できます。

That pause makes the structure easier to read:

• As for this teacher,
• you can consult freely about studying Japanese

In writing, especially when the opening phrase is a setup like この先生なら, a comma is very common and natural.