Cuando la profesora empieza a hablar, nos callamos enseguida.

Because this sentence describes a habitual or general situation: whenever the teacher starts talking, we go quiet.

In Spanish, with cuando, you often use the present tense for repeated actions:

• Cuando la profesora empieza a hablar, nos callamos enseguida.
• Cuando llego a casa, ceno.

This is similar to English when/whenever in general statements.

Also, Spanish often uses the present after cuando even for the future:

• Cuando llegue, te llamo.
  = When he/she arrives, I’ll call you.

So the present after cuando is very common.

Here, la profesora means the teacher: a specific teacher that both speaker and listener can identify.

Spanish usually uses the definite article when referring to a specific person by role or title:

• La profesora entra en clase.
• El médico ya viene.

But after ser, professions normally appear without an article:

• Es profesora.
• Soy médico.

So:

• la profesora = the teacher
• es profesora = she is a teacher

Because empieza is the 3rd person singular present form of empezar, matching la profesora.

Conjugation:

• yo empiezo
• tú empiezas
• él/ella/usted empieza
• nosotros/nosotras empezamos
• vosotros/vosotras empezáis
• ellos/ellas/ustedes empiezan

A useful thing to notice: empezar is a stem-changing verb, so e → ie in most present-tense forms:

• empiezo, empiezas, empieza, empiezan
• but empezamos, empezáis

Because empezar is commonly followed by a + infinitive when it means to begin/start doing something.

So:

• empezar a hablar = to start speaking
• empezar a estudiar = to start studying
• empezar a correr = to start running

This is a fixed pattern:

• empezar + a + infinitive

You cannot normally leave out the a here.

In standard Spanish, the h is not pronounced.

So hablar sounds roughly like ablar.

Examples:

• hablar
• hacer
• hola

All have a silent h.

This is one of the first pronunciation rules learners notice in Spanish.

Because the verb here is callarse, not just callar.

• callar = to silence someone / to keep something quiet
• callarse = to be quiet / to fall silent / to shut up

So:

• Nos callamos = we become quiet / we fall silent
• Callamos a los niños = we silence the children

In this sentence, the reflexive/pronominal form is the natural one because the subject is the group itself becoming quiet.

Nos is the reflexive pronoun for we / ourselves, matching nosotros/nosotras.

In nos callamos:

• nos = ourselves
• callamos = we become quiet / we quiet ourselves

It does not usually need to be translated literally as ourselves in English, but grammatically that is the idea.

Compare:

• me callo = I go quiet
• te callas = you go quiet
• se calla = he/she goes quiet
• nos callamos = we go quiet
• os calláis = you all go quiet
• se callan = they go quiet

It is a pronominal verb, and in practice learners often treat it like a reflexive verb because it uses reflexive pronouns:

• me, te, se, nos, os, se

But semantically it is not always a literal action done to oneself in the same way as:

• lavarse = to wash oneself

With callarse, the pronoun is just part of the normal verb form used to mean to be quiet / to fall silent.

So for learning purposes, it is best to remember:

• callar = to silence / to keep quiet about something
• callarse = to become quiet, to shut up

Enseguida means immediately, right away, or straight away.

In this sentence, it shows that the reaction is very quick:

• nos callamos enseguida = we go quiet right away

It is very common in everyday Spanish.

Similar expressions:

• inmediatamente = immediately, more formal
• al momento = right away
• en el acto = on the spot, immediately

In Spain, enseguida is extremely common in speech.

The comma separates the time clause from the main clause:

• Cuando la profesora empieza a hablar,
• nos callamos enseguida.

This is very natural when the sentence begins with a subordinate clause.

English does the same quite often:

• When the teacher starts speaking, we go quiet immediately.

If the main clause came first, the comma might often be omitted:

• Nos callamos enseguida cuando la profesora empieza a hablar.

That version is grammatically possible, though the original word order sounds more natural for highlighting the sequence.

Yes. Comenzar and empezar both mean to begin/start.

So you could say:

• Cuando la profesora comienza a hablar, nos callamos enseguida.

Both are correct. In many everyday contexts, empezar is a bit more common in speech, but both are very normal.

Also note that comenzar is another stem-changing verb:

• comienza
• comienzan
• but comenzamos

Because the sentence describes a regular pattern or classroom routine.

It is basically saying:

• Whenever the teacher starts speaking, we immediately go quiet.

For habits and general truths, Spanish uses the present tense in both clauses:

• Cuando llueve, me quedo en casa.
• Cuando suena el teléfono, contesta ella.

If you wanted a past habit, you would usually switch to the imperfect:

• Cuando la profesora empezaba a hablar, nos callábamos enseguida. = Whenever the teacher started speaking, we would go quiet immediately.

Grammatically, it is cuando, but in English the best translation depends on context.

Because the sentence describes a repeated situation, English often naturally uses whenever:

• Whenever the teacher starts speaking, we go quiet right away.

But when is also possible:

• When the teacher starts speaking, we go quiet right away.

So in this kind of sentence, cuando can correspond to either when or whenever, depending on how you express the habitual sense in English.

In Spain, a few pronunciation points stand out:

• c in profesora is not present, so nothing special there.
• z in empieza is pronounced with the th sound of English thin in most of Spain: em-pye-tha
• h in hablar is silent
• ll in callamos is usually pronounced like y in most of Spain today: roughly ka-ya-mos

A broad, learner-friendly approximation for Spain would be:

• KWAN-do la pro-fe-SO-ra em-PYE-tha a a-BLAR, nos ka-YA-mos en-se-GEE-da

That said, pronunciation varies by region, and many speakers pronounce ll and y exactly the same.