Mi preferas uzi karton, sed mia fratino ĉiam portas iom da kontanto kaj kelkajn monerojn.

Because preferi is followed by another verb in the infinitive when the same person does both actions.

Here, mi preferas uzi karton means I prefer to use a card. The subject mi is both the one who prefers and the one who uses.

This is very similar to English:

• I prefer to use...
• Esperanto: Mi preferas uzi...

If the subject changed, Esperanto would usually use a clause with ke instead.

The -n marks the direct object.

In Mi preferas uzi karton, the thing being used is a card, so karton is the object of uzi.

A very important Esperanto rule is:

• subject: no -n
• direct object: -n

So:

• karto = a card
• karton = a card as the direct object

English mostly shows this through word order, but Esperanto marks it directly.

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

In mia fratino ĉiam portas..., the sister is the one doing the carrying, so she is the subject and does not take -n.

Also, mia is a regular adjective in Esperanto, so it agrees with the noun when necessary. That means:

• mia fratino = my sister
• mian fratinon = my sister as a direct object

Since this sentence needs the subject form, mia fratino is correct.

Porti usually means to carry, to wear, or to have on one’s person.

In this sentence, portas means something like:

• carries
• keeps with her
• has on her

So mia fratino ĉiam portas iom da kontanto kaj kelkajn monerojn means that she regularly has cash and some coins with her.

This is broader than just physically holding something in the hand.

Ĉiam is an adverb meaning always.

Its position is fairly flexible, but putting it before the verb is very natural:

• mia fratino ĉiam portas...

That is similar to English my sister always carries...

You could move it for emphasis, but this placement is the most straightforward and idiomatic for learners.

Iom da means some amount of.

It is used especially with things that are treated as uncountable or when you do not want to give a precise quantity.

So:

• iom da kontanto = some cash / some amount of cash

This is a very common pattern in Esperanto:

• iom da akvo = some water
• iom da tempo = some time
• iom da mono = some money

The word da links a quantity expression to a noun.

Because kontanto here is part of the expression iom da kontanto.

After da, the following noun normally does not take the accusative -n. The quantity expression itself handles the structure.

Also, kontanto is being treated as an uncountable noun, like English cash, so singular makes sense.

So:

• iom da kontanto = some cash

not

• iom da kontantojn

Mono means money in a general sense.

Kontanto means cash, specifically money in physical form.

That makes kontanto a better choice here, because the sentence contrasts using a card with carrying physical money.

So the contrast is roughly:

• uzi karton = use a card
• porti kontanton = carry cash

Because kelkaj works like an adjective/determiner and must agree with the noun.

Here:

• moneroj is plural, so it gets -j
• the whole phrase is a direct object, so it gets -n
• the agreeing word kelkaj also gets both endings

So:

• kelkaj moneroj = some coins
• kelkajn monerojn = some coins as a direct object

This agreement is very typical in Esperanto.

Because they represent two different kinds of nouns.

Kontanto is treated like a mass noun, similar to English cash. You normally think of it as an amount, not as individual countable items.

Moneroj, on the other hand, are countable individual objects: one coin, two coins, several coins.

So the sentence naturally says:

• iom da kontanto = some cash
• kelkajn monerojn = some coins

Because Esperanto has no indefinite article.

English distinguishes:

• a card
• the card

Esperanto only has a definite article, la, for the. If something is indefinite, you usually say nothing.

So:

• karton = a card
• la karton = the card

In this sentence, the card is not a specific known one, so no article is needed.

Yes, very closely.

Sed is a coordinating conjunction that connects two contrasting ideas:

• Mi preferas uzi karton
• sed mia fratino ĉiam portas...

It works much like English but, and it does not force any special word order changes.

So this is one of the easier Esperanto conjunctions for English speakers.

By itself, karto just means card.

However, in this context—especially because it is contrasted with cash and coins—a learner should understand it as something like:

• bank card
• debit card
• credit card
• payment card

If you wanted to be more specific, Esperanto could use a more exact compound, but karto is enough here because the context makes the meaning clear.