La kontraŭulo asertis tion.

La is the definite article, meaning the.

So la kontraŭulo means the opponent or the adversary, not just an opponent. Esperanto has only one definite article: la. It does not change for gender, number, or case.

Kontraŭulo is a compound word:

• kontraŭ = against, opposite
• -ul- = a person characterized by something
• -o = noun ending

So kontraŭulo literally means something like a person on the opposite side. In natural English, that becomes opponent, adversary, or rival, depending on context.

This is very typical Esperanto word-building: a root or element + a suffix + a part-of-speech ending.

In Esperanto, verbs always show tense with a fixed ending:

• -as = present
• -is = past
• -os = future
• -us = conditional
• -u = imperative / jussive
• -i = infinitive

So asertis means asserted because -is marks the past tense.

The verb stem here is asert-, from aserti = to assert.

Because tion is the direct object of the verb.

In Esperanto, direct objects normally take the ending -n. So:

• tio = that
• tion = that, as a direct object

Since the verb aserti is transitive and something is being asserted, that something is the object, so tion is correct.

Because la kontraŭulo is the subject, not the direct object.

In La kontraŭulo asertis tion:

• la kontraŭulo = the one doing the action
• tion = the thing being asserted

Only the direct object gets -n here, so tion has -n, but kontraŭulo does not.

No, Esperanto word order is fairly flexible, especially because the -n ending helps show what is the object.

So these can all work:

• La kontraŭulo asertis tion.
• Tion asertis la kontraŭulo.

Both mean the same basic thing. The second version puts more emphasis on tion.

That said, subject–verb–object is the most neutral and common order, so La kontraŭulo asertis tion is the straightforward version.

Tion is the pronoun form of that. It stands by itself, without a noun after it.

It usually refers to:

• a previous statement
• an idea
• a fact
• something already understood from context

Compare:

• tion = that
• tiun ideon = that idea
• tiun frazon = that sentence

So tion is used when Esperanto does not need to repeat the noun.

A common pronunciation guide is roughly:

kon-TRAU-lo

A few useful points:

• aŭ is a diphthong, somewhat like ow in cow
• the stress in Esperanto is always on the second-to-last syllable
• so the stress falls on traŭ

Syllable breakdown: kon-traŭ-u-lo

Even in longer words, Esperanto stress stays very regular, which makes pronunciation easier once you know the rule.