Mi estas malsata, sed mia fratino estas soifa kaj volas sukon.

In Esperanto, hungry and thirsty are usually expressed as adjectives, so you say:

• esti malsata = to be hungry
• esti soifa = to be thirsty

So Mi estas malsata literally means I am hungry.

This is very similar to English, which also uses to be + adjective here.

mal- is a very common Esperanto prefix meaning the opposite of.

So:

• sata = full, satiated
• malsata = hungry

This is a very productive pattern in Esperanto. For example:

• bona = good → malbona = bad
• granda = big → malgranda = small
• varma = warm → malvarma = cold

So malsata is built regularly from sata.

Because soifa already means thirsty. It is its own root.

So:

• soifa = thirsty
• malsoifa = not thirsty / the opposite of thirsty

By contrast, hungry is often expressed with malsata, built from sata (full). So the two ideas are formed differently:

• malsata = hungry
• soifa = thirsty

That is just how the vocabulary works.

Esperanto builds many words from roots plus affixes.

Here:

• frato = brother
• -in- = female
• fratino = sister

So fratino is literally female brother/sibling-word, meaning sister.

This is a very common pattern:

• patro = father → patrino = mother
• filo = son → filino = daughter
• onklo = uncle → onklino = aunt

Because mia fratino is the subject of the second clause, not the direct object.

In Esperanto, the -n ending marks a direct object. In this sentence:

• mia fratino = subject
• sukon = direct object

So:

• mia fratino estas soifa kaj volas sukon = my sister is thirsty and wants juice

Since my sister is doing the action, there is no -n.

Also, adjectives and possessives agree with the noun when needed. If fratinon had -n, then mia would stay mia, because correlatives and possessives do not take agreement endings themselves like ordinary adjectives do? Wait: actually mia does behave like an adjective and can take -n and -j. So if it were a direct object, it would be mian fratinon. But here it is not.

Because sukon is the direct object of volas.

In Esperanto, direct objects normally take -n:

• Mi volas sukon = I want juice
• Ŝi trinkas akvon = She drinks water
• Ili vidas la domon = They see the house

In your sentence, the sister is the one who wants something, and the thing wanted is juice, so it becomes sukon.

Because malsata and soifa are predicate adjectives, not direct objects.

After esti (to be), Esperanto uses adjectives to describe the subject:

• Mi estas malsata
• Ŝi estas soifa
• Ili estas lacaj

These adjectives match the subject in number and case only when necessary, but in a normal sentence like this, the subject is not in the accusative, so there is no -n.

Because voli takes a direct object in Esperanto.

So you say:

• voli ion = to want something
• Mi volas kafon = I want coffee
• Li volas helpon = He wants help

No preposition is needed here. The thing wanted is simply the direct object, so it takes -n.

Because there are two different subjects and two separate descriptions:

• Mi estas malsata
• mia fratino estas soifa kaj volas sukon

You could not leave out the second estas, because my sister also needs its own verb.

In Esperanto, repeating the verb like this is completely normal and often necessary for clarity.

sed means but. It connects two clauses with a contrast:

• Mi estas malsata = I am hungry
• sed mia fratino estas soifa = but my sister is thirsty

The contrast is not a strong logical opposition, but simply a difference between the two people and their needs. This is very natural.

kaj means and. Here it links two things about my sister:

• estas soifa = is thirsty
• volas sukon = wants juice

So:

• mia fratino estas soifa kaj volas sukon

means that both are true about her.

Because the subject stays the same, Esperanto does not repeat mia fratino before volas.

Yes. malsati is a verb meaning to be hungry / to hunger, so Mi malsatas is also possible.

Likewise:

• Mi soifas = I am thirsty

However, esti + adjective is very common and often feels straightforward for beginners:

• Mi estas malsata
• Mi estas soifa

Both patterns are correct, though the exact style may differ slightly.

It actually can change when grammar requires it, because mia behaves like an adjective.

Here, fratino is singular nominative, so mia stays mia:

• mia fratino = my sister

If the noun were plural or accusative, mia would match:

• miaj fratinoj = my sisters
• mian fratinon = my sister (as direct object)
• miajn fratinojn = my sisters (as direct object)

In this sentence, none of those extra endings are needed.

In Esperanto, stress is always on the second-to-last syllable.

So here you get:

• MI es-tas mal-SA-ta
• sed MI-a fra-TI-no es-tas so-I-fa
• kaj VO-las SU-kon

That regular stress pattern is one of the helpful features of Esperanto.