Si tienes una duda, la profesora la aclara con calma.

Yes. Si here means if.

So Si tienes una duda... means If you have a question/doubt...

A useful thing to remember is:

• si = if
• sí = yes

The accent makes a big difference.

Because this is a normal, real condition, not a hypothetical or doubtful one.

In Spanish, with sentences like If you have a question, the teacher explains it calmly, the if-clause usually takes the present indicative when talking about something general or likely:

• Si tienes una duda, ...
• Si estudias, aprendes.

You would use subjunctive in other structures, but not in this straightforward si + present pattern.

Yes, both are possible, but they mean slightly different things.

• una duda = a question / a doubt / one doubt
• dudas = questions / doubts in general or more than one

So:

• Si tienes una duda, la profesora la aclara con calma. = If you have a question, the teacher clears it up calmly.
• Si tienes dudas, la profesora las aclara con calma. = If you have questions, the teacher clears them up calmly.

Both sound natural.

In Spanish, it is very common to use the definite article with nouns when talking about someone in a general role:

• la profesora
• el médico
• la directora

Here la profesora means the teacher.

Spanish often uses the article where English might not, especially in general statements.

That la is a direct object pronoun, and it refers back to una duda.

• una duda is feminine singular
• so the pronoun is la

So the structure is:

• Si tienes una duda,
• la profesora la aclara = the teacher clears it up

Spanish often repeats the object with a pronoun instead of leaving it out.

Because Spanish normally needs the direct object to be expressed if you want to say what is being clarified.

• La profesora aclara by itself sounds incomplete: The teacher clarifies... clarifies what?
• La profesora la aclara clearly means The teacher clarifies it

Since la refers to una duda, it works like it in English.

Here aclarar means something like:

• to clear up
• to explain
• to make clear

With una duda, it usually means the teacher helps remove the confusion.

So la aclara is close to:

• clears it up
• explains it
• clarifies it

Because the whole sentence expresses a general truth or habitual action.

It is not talking about one specific moment only. It means something like:

• If you have a question, the teacher explains it calmly
• Whenever you have a question, the teacher clears it up calmly

Spanish often uses the present tense for this kind of general statement.

Con calma means:

• calmly
• patiently
• literally, with calm

It describes the way the teacher explains things.

A very natural translation here is:

• calmly
• patiently

Depending on context, patiently may even sound more natural in English, though the Spanish literally says with calm.

Not exactly, but in many classroom contexts it works that way.

Duda literally means doubt, but in Spanish people often say tener una duda when they mean:

• to have a question
• to be unsure about something

So in a school context, Si tienes una duda... is very naturally understood as If you have a question...

Yes, absolutely.

• una pregunta = a question
• una duda = a doubt / a question about something you do not understand

The difference is subtle:

• pregunta focuses on the act of asking
• duda focuses on uncertainty or confusion

In a classroom, both can be natural, but duda often sounds especially appropriate when the student does not understand something.

Yes, it can be rearranged a little.

For example:

• La profesora aclara la duda con calma si tienes una duda.
  Grammatically possible, but less natural here.

• Si tienes una duda, la profesora te la aclara con calma.
  Very natural, and it adds te = to you / for you

The original sentence is simple and natural because it presents the condition first, then the result.

Because la profesora la aclara con calma is one complete clause.

The comma appears after duda because the sentence begins with the if-clause:

• Si tienes una duda, ← introductory clause
• la profesora la aclara con calma. ← main clause

That punctuation is standard and natural.

Yes, and that is very common.

• la = it, referring to una duda
• te = to you / for you

So:

• La profesora la aclara con calma = The teacher clears it up calmly.
• La profesora te la aclara con calma = The teacher clears it up for you calmly.

The version with te is slightly more explicit about who benefits from the action.