Antaŭ la ekzameno mi atente legas ĉiun noton en mia kajero.

Esperanto -as is the present tense, but it is also often used for:

• habitual actions: something you regularly do
• general statements: what you do in that situation
• vivid present: describing an action as if it is happening now

So this sentence most naturally suggests a habit or routine: before the exam, I carefully read every note in my notebook.

If you wanted a different time, you could change the verb:

• legis = read, used to read
• legos = will read

So the tense depends on the intended context; this sentence uses the present as a neutral, common choice.

After a preposition like antaŭ, Esperanto normally uses the basic form of the noun, not the accusative -n.

So:

• antaŭ la ekzameno = before the exam

The -n after a preposition is mainly used when there is a sense of direction toward a place. But here antaŭ is being used in a time expression, not a direction expression, so no -n is needed.

La means the. It shows that the exam is understood as a specific one.

So la ekzameno suggests:

• the exam we both know about
• the relevant exam in this situation

That is very similar to English. Compare:

• antaŭ la ekzameno = before the exam
• antaŭ ekzameno = before an exam / exam time, more indefinite or general

In this sentence, la makes the exam sound definite and specific.

Because atente is an adverb, and it modifies the verb legas.

• atenta = attentive, careful — an adjective
• atente = attentively, carefully — an adverb

Here we are describing how the reading is done, so Esperanto uses -e:

• mi atente legas = I read carefully

If you said mi estas atenta, then atenta would be correct, because it describes me, not the action.

Because noton is the direct object of legas, and ĉiun agrees with noton.

In Esperanto:

• the direct object takes -n
• words that go together with that noun also match it

So:

• ĉiu noto = each note
• ĉiun noton = each note, as a direct object

This matching is called agreement. Esperanto does it very regularly.

Both can be translated naturally in English, but they are not exactly the same.

• ĉiun noton = each/every note, looking at them one by one
• ĉiujn notojn = all the notes, focusing more on the whole group

So in this sentence, ĉiun noton emphasizes individual notes:

• I carefully read each note

That is a very normal and common use of ĉiu.

En means in or inside. So en mia kajero means the notes are in my notebook.

That fits the sentence well:

• ĉiun noton en mia kajero = every note in my notebook

El means out of / from. If you said el mia kajero, that would shift the idea a little:

• legi el mia kajero = read from my notebook

That is also possible in some contexts, but it focuses more on the notebook as the source you are reading from.
Here, en is the most natural choice if the intended idea is that the notes are located in the notebook.

Because mia kajero is part of the prepositional phrase en mia kajero.

After en, nouns normally do not take -n unless there is movement into something.

So:

• en mia kajero = in my notebook, location
• en mian kajeron = into my notebook, direction

In this sentence, the notes are located in the notebook, so mia kajero is correct.

Yes. Esperanto word order is fairly flexible because endings show the grammatical roles.

For example, these are possible:

• Mi atente legas ĉiun noton en mia kajero antaŭ la ekzameno.
• Atente mi legas ĉiun noton en mia kajero antaŭ la ekzameno.

But the original order is good because it is clear and natural:

• Antaŭ la ekzameno sets the time first
• mi atente legas gives the subject and action
• ĉiun noton en mia kajero gives the object and its location

So word order can change, but it often changes emphasis or style, not the basic meaning.

A helpful rough guide is:

• antaŭ ≈ an-TAU, where aŭ sounds like the ow in now
• ĉiun ≈ CHEE-oon
• kajero ≈ kah-YE-ro

A few useful pronunciation points:

• ĉ sounds like ch in church
• j sounds like English y
• Esperanto stress is usually on the second-to-last syllable

So:

• an-TAŬ
• ĈI-un
• ka-JE-ro