Puer stilum e mensa capit et in charta unam sententiam scribit.

Because puer is in the nominative case, which is the case normally used for the subject.

Also, both verbs — capit and scribit — are 3rd person singular, so they match puer: the boy takes ... and writes ...

A useful point for English speakers: in Latin, the case ending tells you the job of the word more clearly than the word order does.

Because stilum is the direct object of capit.

• stilus = nominative singular, the stylus / pen as subject
• stilum = accusative singular, the stylus / pen as object

So in this sentence, the boy is taking the pen, not being the pen, so Latin uses the accusative: stilum.

Here mensa is in the ablative singular.

That is because the preposition e (or ex) means out of / from, and it takes the ablative case.

So:

• mensa can be nominative: the table
• mensā (usually written without the macron as mensa) can be ablative: from the table, when used with e

In the sentence, e mensa means from the table.

Both e and ex mean the same thing here: out of / from.

A common pattern is:

• e before a consonant
• ex before a vowel or sometimes h

Since mensa begins with m, a consonant, e mensa is the expected form.

You may still see some variation in real Latin, but this is the basic rule learners are usually taught.

Because in can take two different cases with two different meanings:

• in + ablative = in / on a place, showing location
• in + accusative = into / onto a place, showing motion toward it

Here the idea is he writes on the paper, so Latin uses in + ablative:

• in charta = on/in the paper

If the idea were movement onto something, then the accusative would be used.

For English speakers, this is slightly tricky because English often says on the paper, while Latin may use in in places where English prefers on.

Because both words belong together, and both must match in case, number, and gender.

• sententiam is accusative singular feminine
• unam is also accusative singular feminine

So unam agrees with sententiam.

Also, sententiam is the direct object of scribit, so it must be in the accusative.

The phrase means one sentence or sometimes simply a single sentence, depending on context.

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

So:

• puer can mean the boy, a boy, or simply boy, depending on context
• stilum can mean the pen or a pen
• sententiam can mean the sentence or a sentence

English has to choose an article when translating, but Latin usually leaves that idea to context.

It matters much less than in English.

Latin word order is flexible because the endings show the grammatical roles. So even if the words were rearranged, the cases would still tell you who is doing what.

This sentence has a very natural order:

• Puer — subject first
• stilum — object of capit
• e mensa — where he takes it from
• capit
• et
• in charta
• unam sententiam
• scribit

Latin often puts the verb later in the clause, sometimes at the end, but that is a tendency, not a strict rule.

Because the same subject continues for both verbs.

So Latin says:

• Puer ... capit et ... scribit
  = The boy takes ... and writes ...

There is no need to repeat puer, because:

1. the conjunction et links the two actions, and
2. scribit is already 3rd person singular, so it naturally refers back to puer.

English works the same way: we usually say The boy takes the pen and writes a sentence, not The boy takes the pen and the boy writes a sentence.

You know mainly from sense and from the way the sentence is grouped.

• stilum e mensa capit = he takes the pen from the table
• in charta unam sententiam scribit = he writes one sentence on the paper

Latin does allow flexible word order, but it still usually keeps related words reasonably close together. Also, the meaning helps:

• you normally take a pen
• you normally write a sentence

So even without English-style fixed order, the relationships are clear.

It can mean several related things, including takes, seizes, gets, or picks up, depending on context.

In this sentence, since the boy is taking the pen from the table, English might naturally say picks up the pen. But takes the pen is also a perfectly good translation.

This is common in Latin: one verb often covers a range that English may express with several different verbs.