Lia taktiko surprizis la kontraŭulon.

Esperanto shows this with the direct-object ending -n.

• Lia taktiko has no -n, so it is the subject: the thing doing the action.
• la kontraŭulon has -n, so it is the direct object: the one receiving the action.

So the tactic did the surprising, and the opponent was surprised.

The ending -is marks the past tense in Esperanto.

So:

• surprizas = surprises / is surprising
• surprizis = surprised
• surprizos = will surprise

A useful point for English speakers: Esperanto verbs do not change for person or number. Mi surprizis, li surprizis, ili surprizis all use the same past-tense ending -is.

Because sia is reflexive: it refers back to the subject of the clause.

In this sentence, the grammatical subject is taktiko, not li. So sia taktiko would mean something like its own tactic, referring back to taktiko, which is not what is meant here.

So Esperanto uses lia to show that the tactic belongs to some male person, not to the subject itself.

Compare:

• Lia taktiko surprizis la kontraŭulon. = His tactic surprised the opponent.
• Li surprizis la kontraŭulon per sia taktiko. = He surprised the opponent with his own tactic.

In the second sentence, the subject is li, so sia works.

Because possessives like lia already make a noun definite.

So lia taktiko already means his tactic, which is specific enough by itself. Esperanto normally does not add la there.

But la kontraŭulon uses la because it means a specific, known opponent.

Also, Esperanto has no separate word for a or an, so nouns without la are often just indefinite or general depending on context.

It is a built word, which is very common in Esperanto.

• kontraŭ = against
• -ul- = a person characterized by something
• -o = noun ending
• -n = direct-object ending

So:

• kontraŭulo = an opponent, adversary, one who is against someone
• kontraŭulon = opponent, as the direct object

This kind of word-building is one of the most useful things to get comfortable with in Esperanto.

Yes. It is completely regular.

The root is surpriz-, and then you add the normal verb endings:

• surprizi = to surprise
• surprizas = surprises / is surprising
• surprizis = surprised
• surprizos = will surprise
• surprizu = surprise! / should surprise
• surprizus = would surprise

Unlike English, you do not have irregular forms like surprized versus something else. The system stays consistent.

Yes, more easily than in English, because the -n ending marks the direct object.

The neutral order here is:

• Lia taktiko surprizis la kontraŭulon.

But you could also say:

• La kontraŭulon surprizis lia taktiko.

That is still grammatical, because kontraŭulon still has -n, so it is still the object. Changing the order usually changes emphasis rather than the basic meaning.

Yes. Lia means his, referring to one male person.

Related forms are:

• ŝia = her
• ilia = their
• ĝia = its
• sia = one’s own, referring back to the subject

So lia taktiko specifically says that the tactic belongs to a male person.

A helpful breakdown is:

• kon-traŭ-u-lon

A few pronunciation points:

• aŭ sounds roughly like the ow in cow
• the stress in Esperanto is always on the next-to-last syllable
• so the stress here falls on u: kon-traŭ-U-lon

If you follow Esperanto spelling closely, pronunciation becomes very predictable.