Mi ĉiam uzas sekurzonon, se mi sidas en aŭto, sed sur biciklo mi portas kaskon.

Because they are direct objects of the verbs.

• uzi = to use
• porti = to wear, carry

In Esperanto, the direct object usually takes the accusative ending -n.

So:

• Mi uzas sekurzonon = I use a seat belt
• Mi portas kaskon = I wear a helmet

The subject mi does not get -n. Only the thing directly affected by the verb does.

Esperanto follows the same basic idea as English here:

• en aŭto = in a car
• sur biciklo = on a bicycle

You are thought of as being inside a car, but on top of / on a bicycle.

This is a very common preposition pattern with vehicles in Esperanto.

Because each clause normally states its subject clearly.

The sentence has three clauses:

• Mi ĉiam uzas sekurzonon
• se mi sidas en aŭto
• sed sur biciklo mi portas kaskon

In Esperanto, you usually do not leave out the subject the way English sometimes can in coordinated ideas. Repeating mi keeps everything clear and natural.

Ĉiam means always. It modifies the action uzas.

So:

• Mi ĉiam uzas sekurzonon = I always use a seat belt

Placing ĉiam before the verb is very normal. Esperanto word order is flexible, but this placement is clear and common.

You could also see:

• Ĉiam mi uzas sekurzonon

but that gives a slightly different emphasis, with more focus on always.

Yes, se normally means if. Here it introduces a condition:

• se mi sidas en aŭto = if I am sitting in a car

In a sentence about a habit, English often uses if in a similar way:

• I always wear a seat belt if I am in a car.

This can feel close to whenever. Esperanto works the same way here.

If you used kiam, that would mean when, which is also possible in some contexts, but se emphasizes the condition: whenever that situation applies, the action happens.

Yes, literally it means sit in a car. Esperanto often uses simple, concrete wording like this.

• sidi = to sit
• en aŭto = in a car

So se mi sidas en aŭto literally means if I am sitting in a car.

In natural use, this can refer to being in the car as an occupant, not just emphasizing the physical act of sitting down. Esperanto often allows this kind of straightforward expression.

Depending on context, other verbs could also be used, but this sentence is perfectly normal.

Because the two objects are treated differently.

• A seat belt is something you use: uzi sekurzonon
• A helmet is something you wear: porti kaskon

Porti can mean carry or wear, depending on context. With clothing or gear on your body, it usually means wear.

So the sentence uses the most natural verb for each item.

Because Esperanto uses la only for the, and it has no indefinite article like English a/an.

So:

• aŭto can mean a car
• biciklo can mean a bicycle
• kasko can mean a helmet

Also, this sentence is talking about things in general, not one specific known car, bicycle, seat belt, or helmet. That is another reason la is unnecessary.

Esperanto word order is fairly flexible. Here, sur biciklo is placed first for contrast:

• in a car → seat belt
• on a bicycle → helmet

So fronting sur biciklo helps set up that contrast after sed.

Both of these are possible:

• sed sur biciklo mi portas kaskon
• sed mi portas kaskon sur biciklo

The version in the sentence sounds a bit more balanced and contrastive.

Yes. Esperanto builds many words by combining meaningful parts.

• sekur- = safe, secure
• zono = belt

So sekurzono is literally something like safety-belt or secure-belt, which corresponds to seat belt.

This is a very typical Esperanto feature: instead of memorizing a completely unrelated word, you can often understand it from its parts.

The letter ĉ is pronounced like ch in church.

So:

• ĉiam sounds roughly like CHEE-am

A few useful pronunciation notes from this sentence:

• ĉ = English ch
• aŭ sounds like ow in now
• stress in Esperanto normally falls on the second-to-last syllable

So:

• ĉi-am
• aŭ-to
• bi-ci-klo
• se-kur-ZO-no
• KAS-kon

That regular pronunciation system is one of Esperanto’s easiest features to learn.