La studentessa mi guarda come se sapesse già la risposta, ma poi sorride.

In Italian, it is very common to use the definite article with nouns where English might not.

• la studentessa = the female student
• Without the article, studentessa would usually sound less natural here unless it appeared in a special context.

Also, studentessa is the feminine form of studente. So the sentence is specifically talking about a female student.

Mi is an object pronoun meaning me, and in Italian unstressed object pronouns usually come before the conjugated verb.

So:

• mi guarda = she looks at me
• literally: me she-looks-at

This is normal Italian word order for pronouns:

• mi vede = she sees me
• ti ascolta = she listens to you
• ci chiama = she calls us

Here guarda means looks at.

The verb is guardare, which often means:

• to look at
• to watch

So:

• mi guarda = she looks at me

This is different from vedere, which usually means to see.
A useful contrast:

• mi guarda = she is directing her eyes toward me
• mi vede = she sees me

Because come se normally requires the subjunctive in Italian.

So:

• come se sapesse = as if she knew

Even if English often uses a past form like knew, Italian uses the imperfetto congiuntivo here:

• sapesse = imperfect subjunctive of sapere

This structure is very common:

• parla come se fosse il capo = he speaks as if he were the boss
• mi guarda come se capisse tutto would be wrong in standard Italian
• correct: mi guarda come se capisse tutto

After come se, Italian usually uses:

• imperfetto congiuntivo for a situation imagined as simultaneous or unreal in the present/past context
• trapassato congiuntivo for something imagined as having happened earlier

Here, the idea is:

• she looks at me as if she already knew the answer

That is why Italian uses sapesse.

Compare:

• come se sapesse = as if she knew
• come se avesse saputo = as if she had known

Sapesse is the third person singular imperfect subjunctive of sapere.

The imperfect subjunctive forms of sapere are:

• io sapessi
• tu sapessi
• lui/lei sapesse
• noi sapessimo
• voi sapeste
• loro sapessero

So in this sentence, since the subject is la studentessa, we use sapesse.

Già means already.

So:

• sapesse già la risposta = already knew the answer

It adds the idea that she seems to know the answer beforehand.

Its position is normal here: it comes before the noun phrase it modifies with the verb idea.

You could think of the phrase as:

• sapesse già = already knew
• la risposta = the answer

La risposta means the answer, so it refers to a specific answer, probably the one to a question already understood in context.

If you said una risposta, it would mean an answer, which is less specific.

So the sentence suggests that she seems to know the answer, not just some answer.

Italian often leaves out subject pronouns because the verb ending already shows who the subject is.

So:

• guarda = she/he looks
• sorride = she/he smiles

Since the subject la studentessa is already stated, there is no need to repeat lei.

Italian does this much more than English:

• La studentessa mi guarda... poi sorride.
• literally: The student looks at me... then smiles.

Poi means then or afterward.

So:

• ma poi sorride = but then she smiles

It shows a change or next step in the situation:

1. she looks at me as if she already knew the answer
2. but then she smiles

The ma adds contrast, and poi adds sequence.

Because once the subject is clear, Italian usually does not repeat it unless there is a reason for emphasis or contrast.

So ma poi sorride sounds natural and smooth.

If you said ma poi la studentessa sorride, it would still be grammatical, but it would sound more marked, as if you wanted to re-emphasize the student.

Yes, but it would change the feel slightly.

• mi guarda = she looks at me / she is looking at me
  This is the simple present, and in Italian it often covers what English expresses with either looks or is looking.
• mi sta guardando = she is looking at me
  This makes the ongoing action more explicit.

In many cases, Italian prefers the simple present where English might use the progressive. So mi guarda is very natural.

Yes, studentessa is a normal and common word for a female student.

You will often see:

• studente = male student
• studentessa = female student

In some modern discussions about language, people may talk about gender-inclusive alternatives in certain contexts, but in ordinary standard Italian, studentessa is completely normal.

Yes, the main sentence is in the present:

• guarda = looks
• sorride = smiles

The form sapesse is not a normal past tense here. It is the imperfect subjunctive, used because of come se.

So although it may resemble a past form in English translation (as if she knew), its job here is grammatical, not simply to place the action in the past.