Portante ĉi tiun sakon, mi sentas doloron en la brako.

Portante is the active present participle of porti (to carry). In this sentence, it means while carrying or when carrying.

So:

• mi portas ĉi tiun sakon = I carry/am carrying this bag
• portante ĉi tiun sakon = while carrying this bag

Esperanto often uses participles to make a short adverbial phrase. In English, we do something similar with carrying this bag in a sentence like Carrying this bag, I feel pain in my arm.

The person carrying the bag is mi (I).

In Esperanto, a participial phrase like Portante ĉi tiun sakon normally refers to the subject of the main clause. So the sentence means:

While I am carrying this bag, I feel pain in the arm.

It would sound wrong if the participle referred to someone else. In other words, portante must match the doer of sento here, which is mi.

Because sakon is the direct object of portante / porti.

The base noun is sako (bag). Esperanto adds -n to show the direct object:

• sako = a bag
• sakon = a bag as the thing being carried

So:

• porti sakon = to carry a bag
• portante ĉi tiun sakon = while carrying this bag

For the same reason: it is the direct object of senti (to feel).

• doloro = pain
• doloron = pain as the thing felt

So:

• mi sentas doloron = I feel pain

This is very normal Esperanto usage. The verb senti usually takes a direct object for what is felt.

Ĉi adds the idea of this, as opposed to that.

• tiu sako = that bag
• ĉi tiu sako or tiu ĉi sako = this bag

In the sentence, because sakon is accusative, the whole noun phrase changes accordingly:

• ĉi tiu sako = this bag
• ĉi tiun sakon = this bag as object

Both ĉi tiu and tiu ĉi are correct. Ĉi tiu is probably the more common order for many learners to notice first.

Because en la brako describes location, not movement toward somewhere.

In Esperanto:

• en la brako = in the arm
• en la brakon = into the arm

Here the pain is located in the arm, so there is no directional meaning. That is why brako stays without the accusative -n.

Esperanto often uses the body-part style where English might use a possessive.

So Esperanto can say:

• mi sentas doloron en la brako
• literally: I feel pain in the arm

But the meaning is naturally understood as my arm, because it is the speaker who feels the pain.

This is common with body parts and clothing, especially when the owner is obvious from context. Still, en mia brako is also possible if you want to be more explicit.

En la brako is the natural choice if you mean the pain is inside the arm or in the arm area.

Compare:

• en la brako = in the arm
• sur la brako = on the arm, on the surface of the arm
• je la brako = at the arm; possible in some contexts, but less precise here

So if you mean an internal ache from carrying the bag, en la brako fits well.

Yes. A very straightforward alternative is:

Kiam mi portas ĉi tiun sakon, mi sentas doloron en la brako.

That means the same basic thing: When/while I carry this bag, I feel pain in my arm.

The version with portante is a bit more compact and elegant. The version with kiam may feel easier for beginners because it uses a full clause.

Yes. Esperanto word order is fairly flexible, so these are all possible:

• Portante ĉi tiun sakon, mi sentas doloron en la brako.
• Mi, portante ĉi tiun sakon, sentas doloron en la brako.
• Mi sentas doloron en la brako portante ĉi tiun sakon.

The first version is very clear and natural. Moving the participial phrase later in the sentence is possible, but sometimes it can be slightly less clear or elegant depending on context.

Yes. The main tense of the sentence is shown by sentas, which is present tense.

Portante is a participle, not a finite verb. The -ant- part tells you the carrying is ongoing/present relative to the main action. So the sense is:

While carrying this bag, I feel pain in the arm.

If you changed the main verb, the time relationship would shift accordingly:

• Portante ĉi tiun sakon, mi sentas... = while carrying, I feel...
• Portante ĉi tiun sakon, mi sentis... = while carrying, I felt...

Yes, it is natural and grammatical.

It clearly means that carrying the bag causes or accompanies pain in the arm. A few natural alternatives are also possible, depending on style:

• Kiam mi portas ĉi tiun sakon, mi sentas doloron en la brako.
• Portante ĉi tiun sakon, mi sentas doloron en mia brako.
• Ĉi tiu sako dolorigas mian brakon, kiam mi portas ĝin.
  This changes the wording a bit and sounds more like This bag makes my arm hurt when I carry it.

But the original sentence is perfectly good Esperanto.

Doloro is a noun: pain.
Doloras is a verb: hurts / is painful.

So:

• Mi sentas doloron en la brako. = I feel pain in my arm.
• Mia brako doloras. = My arm hurts.

Both are natural, but they focus on slightly different things:

• senti doloron focuses on the experience of pain
• dolori focuses on the body part hurting

So the sentence could also be rephrased as:

Portante ĉi tiun sakon, mia brako doloras.

But that shifts the grammatical subject from I to my arm.