Mi sobrina no siempre obedece a la profesora, pero normalmente se porta bien en clase.

Because Spanish uses the personal a before a person who is the object of the verb.

Here, la profesora is the person being obeyed, so Spanish says:

obedecer a la profesora

Even though English does not use to, this a does not mean the same thing as English to here. It is just a normal Spanish grammar feature with people.

Compare:

• Obedece a su madre. = He/She obeys his/her mother.
• Obedece las reglas. = He/She obeys the rules.

With a person, a is very common.

No. In this sentence, la profesora is still the direct object of obedecer.

The a can be confusing for English speakers, because it looks like an indirect object marker, but here it is simply the personal a.

So:

• Mi sobrina obedece a la profesora. = My niece obeys the teacher.

The teacher is the one being obeyed, so she is the direct object.

Because the subject is mi sobrina, which is third person singular: she.

So the present tense of obedecer here is:

• yo obedezco
• tú obedeces
• él/ella obedece
• nosotros obedecemos
• vosotros obedecéis
• ellos/ellas obedecen

Since mi sobrina = ella, the correct form is obedece.

Because no siempre is the normal way to say not always in Spanish.

• No siempre obedece = She does not always obey.

Siempre no is not the usual neutral phrasing. It can sound odd or very marked depending on context.

So for ordinary statements:

• no siempre = not always
• siempre = always
• normalmente = normally / usually

Portarse bien means to behave well.

So:

• se porta bien = he/she behaves well
• me porto bien = I behave well
• te portas bien = you behave well

It is a very common expression, especially for talking about children.

You can also say:

• portarse mal = to behave badly

So in this sentence:

normalmente se porta bien en clase = she usually behaves well in class

Because portarse is a pronominal verb. The se is part of the verb expression.

This is important because:

• portar by itself usually means to carry or to wear
• portarse means to behave

So:

• porta una mochila = he/she carries a backpack
• se porta bien = he/she behaves well

You should learn portarse bien / mal as a set expression.

En clase is the usual idiomatic way to mean in class / during class in a general sense.

So:

• se porta bien en clase = she behaves well in class

If you say en la clase, it sounds more specific, like:

• in the specific class
• in the classroom
• in that particular lesson

In many cases, Spanish leaves out the article in fixed expressions like this:

• en clase
• en casa
• de noche

They are similar in topic, but not the same in meaning.

• no siempre = not always
• normalmente = normally / usually

So the sentence says:

• she doesn’t always obey the teacher
• but she usually behaves well in class

This creates a nice contrast: she may not obey every time, but overall her behaviour is generally good.

Yes, pero introduces a contrast.

The contrast is:

• not always obeying the teacher sounds negative
• usually behaving well sounds more positive

So pero works like but in English:

She doesn’t always obey the teacher, but she usually behaves well in class.

The two ideas are not complete opposites, but they pull in different directions, which is exactly why pero fits well.

Because mi does not change for masculine vs feminine singular nouns.

So:

• mi sobrino = my nephew
• mi sobrina = my niece

For plural, it becomes mis:

• mis sobrinos
• mis sobrinas

So mi works for both sobrino and sobrina.

Yes. Profesora is the feminine form, so it refers to a female teacher.

• el profesor = the male teacher
• la profesora = the female teacher

In Spain, using the gendered profession noun like this is completely normal.

Because the sentence describes habitual actions, not a single event happening right now.

Spanish often uses the simple present for routines, habits, and general truths.

So here:

• no siempre obedece = she does not always obey
• normalmente se porta bien = she usually behaves well

This is the same idea as English using the present simple for habits:

• She doesn’t always obey
• She usually behaves well