Puer placentam eligere vult, sed soror pirum vult.

Because after vult Latin often uses an infinitive to say wants to do something.

So:

• vult = wants
• eligere = to choose

Together, eligere vult means wants to choose.

This is a very common Latin pattern:

• laborare vult = he/she wants to work
• venire vult = he/she wants to come

Placentam is the direct object of eligere: it is the thing being chosen.

The noun is placenta and here it appears in the accusative singular, which is the case normally used for a direct object.

So:

• placenta = nominative singular
• placentam = accusative singular

A native English speaker often has to get used to the idea that Latin shows a noun’s job in the sentence by its ending.

Because pirum belongs to a different declension and gender from placenta.

• placenta is a first-declension feminine noun, so its accusative singular is placentam
• pirum is a second-declension neuter noun

For neuter nouns in Latin, the nominative and accusative singular are the same. So:

• pirum = nominative singular
• pirum = accusative singular

That is why the object pirum does not change its form here.

They are in the nominative case, which is the case usually used for the subject.

So:

• puer = the boy as subject
• soror = the sister as subject

Also, the verb vult is singular, which matches each singular subject.

Meanwhile:

• placentam is an object
• pirum is an object

Latin relies much more on case endings than English does.

It is repeated because there are really two separate clauses:

• Puer placentam eligere vult
• sed soror pirum vult

Each clause has its own subject and its own verb.

It is not volunt because puer is singular and soror is also singular.
So each clause needs vult = he/she wants, not volunt = they want.

Also, vult can mean:

• wants to ... when followed by an infinitive
• wants ... when followed by a direct object

Because as written, only the first clause explicitly says wants to choose.

• puer placentam eligere vult = the boy wants to choose a cake
• soror pirum vult = the sister wants a pear

In the second clause, vult is being used directly with an object, not with an infinitive.

If you wanted to say the sister wants to choose a pear, you would normally say:

• soror pirum eligere vult

So the Latin sentence is making a real grammatical difference between the two clauses.

Latin word order is much freer than English word order because Latin uses endings to show grammatical function.

A very common Latin pattern is to place the main verb near the end of the clause, so:

• Puer placentam eligere vult
• soror pirum vult

feel natural in Latin.

English depends heavily on word order, but Latin does not depend on it as much.
The same basic meaning could often be expressed with a different order, though some orders sound more natural or give different emphasis.

Latin does not have articles like English the and a/an.

So:

• puer can mean the boy or a boy
• placentam can mean the cake or a cake
• pirum can mean the pear or a pear

You decide from the context which English article makes the most sense.

Vult is the third-person singular present tense of velle, the irregular verb meaning to want.

So:

• vult = he wants, she wants, or it wants

That is why the same form works with both:

• puer ... vult
• soror ... vult

A learner should notice that velle is irregular, so its forms do not look exactly like the forms of regular verbs.

Sed means but.

It joins the two clauses and shows a contrast:

• the boy wants one thing
• but the sister wants another

So sed is a coordinating conjunction, just like English but.