Li sentas sin preta por la ekzameno.

Because the subject and the object are the same person.

• Li = he
• sentas = feels
• sin = himself / oneself in a reflexive sense

So Li sentas sin preta literally means He feels himself ready.

In Esperanto, when someone does something to themselves, you usually use the reflexive pronoun si (or sin when it is a direct object). This is very common after senti when talking about a person’s state:

• Mi sentas min laca. = I feel tired.
• Ŝi sentas sin feliĉa. = She feels happy.
• Ili sentas sin fortaj. = They feel strong.

Because sin is the accusative form.

Here, the reflexive pronoun is the direct object of sentas, so it needs -n:

• si = reflexive pronoun
• sin = reflexive pronoun in the accusative

Compare:

• Li vidas sin. = He sees himself.
• Li parolas pri si. = He talks about himself.

After a preposition like pri, you use si. As a direct object, you use sin.

Normally, no. The usual Esperanto pattern is senti sin + adjective.

So:

• Li sentas sin preta. = natural
• Li sentas preta. = not the normal way to say it

Esperanto usually keeps the idea of feeling oneself to be in a certain state. That is why sin appears.

Because preta describes him, not the way he feels.

In other words, preta is a predicate adjective. It tells you what state the person is in.

• Li sentas sin preta. = He feels ready.
• Li sentas sin laca. = He feels tired.

If you used -e, it would describe the manner of the verb, which is not what is meant here.

So preta agrees in idea with the person, not with the action.

Because preta is not the direct object. The direct object is sin.

In this sentence:

• sentas = verb
• sin = direct object
• preta = predicate adjective describing sin

Predicate adjectives do not take -n just because there is an object in the sentence.

So:

• Li sentas sin preta. = correct
• Li sentas sin preton. = incorrect

If the subject were plural, the adjective would become plural too:

• Ili sentas sin pretaj. = They feel ready.

Because preta matches the person that sin refers to.

Even though sin itself does not change for gender, the adjective still reflects the real subject:

• Li sentas sin preta. = He feels ready.
• Ŝi sentas sin preta. = She feels ready.
• Ili sentas sin pretaj. = They feel ready.

So the adjective agrees with the person being described, even if sin itself stays the same form apart from accusative -n.

Preta por means ready for.

It is a normal Esperanto combination:

• preta por la vojaĝo = ready for the trip
• preta por labori = ready to work
• preta por la ekzameno = ready for the exam

So in your sentence, por la ekzameno tells us what he feels ready for.

Because it comes after the preposition por.

Normally, nouns after prepositions do not take the accusative ending:

• por la ekzameno = for the exam
• kun amiko = with a friend
• pri la libro = about the book

The -n is already on sin because sin is the direct object of the verb. But ekzameno is part of the prepositional phrase por la ekzameno, so it stays without -n.

Because it usually refers to a specific exam that both speaker and listener can identify.

• la ekzameno = the exam
• ekzameno = an exam / exam in general

So this sentence most naturally suggests a particular exam, such as one already mentioned or one that is obvious from context.

The difference is between subjective feeling and plain state.

• Li estas preta. = He is ready.
• Li sentas sin preta. = He feels ready.

The second sentence emphasizes his own perception. He experiences himself as ready. That does not always mean he really is ready in an objective sense.

So senti sin often adds the idea of personal impression or internal state.

Yes, within its own clause, sin refers back to the subject of that clause.

In your sentence, the subject is Li, so sin refers to li.

That is why:

• Li sentas sin preta. = He feels ready.

But this only works inside the same clause. Esperanto learners often need to remember that si/sin cannot freely refer to just any earlier noun; it specifically points back to the subject of its own clause.