Ĉiu studento malfermas sian kajeron antaŭ la leciono.

Ĉiu means each or every, and it normally goes with a singular noun.

So:

• ĉiu studento = each student / every student
• ĉiuj studentoj = all the students

This sentence is looking at the students one by one, so ĉiu studento is the right form.

Sia is the reflexive possessive pronoun in Esperanto. It means his own, her own, their own, and so on, when the possessor is the subject of the clause.

Here, the subject is ĉiu studento, so sian kajeron means their own notebook.

You would use lian, ŝian, or ilian only if the notebook belonged to someone else, not to the student who is opening it.

Because sian describes kajeron, and kajeron is in the accusative.

In Esperanto, a word like sia agrees with the noun it modifies in number and case. So:

• sia kajero = own notebook
• sian kajeron = own notebook, as a direct object

The -n on sian matches the -n on kajeron.

The ending -n marks the direct object in Esperanto.

In this sentence:

• ĉiu studento is the subject
• malfermas is the verb
• kajeron is the thing being opened

So kajeron gets the accusative ending -n.

Yes. Malfermi is built from:

• fermi = to close
• mal- = the opposite

So malfermi literally means to un-close, which in normal English is to open.

This is a very common Esperanto pattern. For example:

• bona = good
• malbona = bad
• granda = big
• malgranda = small

The ending -as is the Esperanto present tense.

Here it expresses a general or habitual action:

• each student opens their notebook before the lesson

Esperanto often uses the simple present in the same kinds of situations where English does, especially for routines or general statements.

Because antaŭ is a preposition, and normally the noun after a preposition does not take -n.

So:

• antaŭ la leciono = before the lesson

The accusative after a preposition is mainly used to show direction or movement in certain cases, but that is not what is happening here. This phrase is just showing time.

Here it means before in time: before the lesson.

But antaŭ can also mean in front of in space. For example:

• antaŭ la domo = in front of the house

So the same preposition can be temporal or spatial, depending on context.

La is the definite article, roughly equivalent to English the.

In la leciono, the speaker is referring to a specific lesson, or to the lesson understood from context.

But:

• ĉiu studento already means each/every student, so no article is needed
• sian kajeron means their own notebook, and possessed nouns in Esperanto often do not need la

So the article use in this sentence is normal.

Yes. Studento is normally gender-neutral in Esperanto unless context makes the gender clear.

If you specifically want to say female student, you can use studentino.

So in this sentence, studento can refer to any student, regardless of gender.