Mi hija masticaba muy despacio porque le dolía un poco la boca.

Masticaba is the imperfect tense, which is often used for an action that was ongoing, habitual, or in progress in the past.

Here, Mi hija masticaba muy despacio gives the idea that she was chewing very slowly over a period of time, not just that she chewed once and finished.

If you said masticó, that would be the preterite, which usually presents the action as a completed whole:

• Masticaba muy despacio = she was chewing very slowly / she used to chew very slowly
• Masticó muy despacio = she chewed very slowly (seen more as a finished event)

In this sentence, the imperfect fits well because it describes the background situation, while le dolía is also imperfect and gives the reason for that ongoing situation.

Le refers to mi hija.

The verb doler works a bit like gustar:

• the thing that hurts becomes the grammatical subject
• the person who feels the pain is expressed with an indirect object pronoun

So:

• Le dolía la boca literally works like The mouth was hurting to her
• natural English: Her mouth hurt / She had a sore mouth

Structure:

• le = to her
• dolía = was hurting
• la boca = the mouth

This is why Spanish does not say it in the same way English does.

In Spanish, when talking about body parts, Spanish often uses:

• an indirect object pronoun for the person affected
• the definite article (el, la, los, las) for the body part

So Spanish prefers:

• Le dolía la boca instead of
• Le dolía su boca

This is very natural in Spanish because le already tells you whose mouth it is.

Other examples:

• Me duele la cabeza = My head hurts
• Le duelen los pies = His/Her feet hurt
• Nos lavamos las manos = We wash our hands

Using su boca is not impossible in every context, but here it sounds less natural.

Because porque here means because and introduces the reason.

The forms are different:

• porque = because
• por qué = why
• el porqué = the reason
• por que = a less common combination in certain structures

In your sentence:

• ...porque le dolía un poco la boca = ...because her mouth hurt a little

If you were asking a question, then you would use por qué:

• ¿Por qué masticaba muy despacio? = Why was she chewing very slowly?

Despacio is an adverb meaning slowly. It is extremely common and natural in everyday Spanish.

So:

• masticaba muy despacio = she was chewing very slowly

A few useful comparisons:

• despacio = slowly
• lentamente = slowly, but often a bit more formal or literary
• lento / lenta = slow, but this is usually an adjective, not an adverb

So:

• muy despacio ✅
• lentamente ✅
• muy lento ❌ here, because masticar needs an adverb, not an adjective

You could say:

• Masticaba lentamente but muy despacio sounds very natural and conversational.

Un poco means a little and here it modifies the idea of pain:

• le dolía un poco la boca = her mouth hurt a little

Its position is natural because it goes with the verb phrase dolía.

Spanish word order is often flexible, but this version sounds very normal:

• Le dolía un poco la boca

You may also hear:

• Le dolía la boca un poco

But the original sentence is smoother and more common.

Both verbs describe an ongoing situation in the past.

• masticaba describes what she was doing
• dolía explains the background cause

So the sentence paints a scene:

• she was chewing slowly
• because her mouth was hurting

This is a classic use of the imperfect in Spanish: describing background actions, states, and causes in the past.

If you changed one or both to the preterite, the meaning would shift:

• Mi hija masticó muy despacio porque le dolió un poco la boca sounds less natural for this context, because it treats the pain more as a brief completed event rather than an ongoing condition affecting how she chewed.

In normal use, mi hija means my daughter.

Spanish hija specifically means a female child in the family sense. It does not usually mean just girl. For girl, Spanish would use words like:

• niña = girl
• chica = girl/young woman

So:

• mi hija = my daughter
• mi niña can sometimes mean my little girl, and depending on context it might refer to a daughter, but literally it means my girl/little girl

In this sentence, mi hija clearly means my daughter.

Yes. That is completely correct.

Adding A mi hija makes the indirect object explicit:

• A mi hija le dolía un poco la boca

This can be useful:

• for emphasis
• for clarity
• if the person affected has not already been mentioned

Spanish often uses both the indirect object phrase and the pronoun together:

• A mi hija le dolía la boca
• A Juan le gusta el café

In your original sentence, mi hija is already the subject of masticaba, so just le is enough in the second clause.

It could be, but it would feel a bit different.

Spanish has two common ways to express an ongoing past action:

• masticaba = imperfect
• estaba masticando = was chewing

The imperfect masticaba is often the more natural, simpler choice when describing background action in a past scene.

Compare:

• Mi hija masticaba muy despacio... = natural narrative description
• Mi hija estaba masticando muy despacio... = puts more focus on the action being in progress at that exact moment

Both are possible, but the imperfect is very common in storytelling and description.

La boca normally means the mouth in a general sense. It does not specify exactly which part hurt.

So it could mean pain in:

• the gums
• teeth area
• jaw area inside the mouth
• the mouth generally

If Spanish wanted to be more specific, it could say:

• los dientes = the teeth
• la encía / las encías = the gum / gums
• la mandíbula = the jaw

As it stands, le dolía un poco la boca is general and natural.

Yes, it is very natural in Spain Spanish.

In fact, despacio is especially common in everyday speech in Spain. The whole sentence sounds normal and idiomatic:

• Mi hija masticaba muy despacio porque le dolía un poco la boca.

Nothing in it sounds strange, overly formal, or non-native.

A few useful pronunciation points:

• hija: the h is silent, so it sounds roughly like EE-ha
• j in hija is a strong throaty sound in Spain, not like English j
• dolía has the stress on -lí- because of the accent mark: do-LEE-a
• masticaba is stressed on -ca-: mas-ti-CA-ba
• despacio is stressed on -pa-: des-PA-cio

Also, in Spain:

• c before i in despacio is pronounced like the th in thin in most of Spain

So in much of Spain, despacio sounds roughly like:

• des-PA-thyo

But in many Latin American varieties, it sounds more like:

• des-PA-syo

Both are correct in their respective accents.