Puer saxum movere non potest.

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

• puer = boy
• nominative singular of puer, pueri

So puer is the one doing the action, or in this sentence, the one who is not able to do the action.

Because saxum is the direct object, not the subject. It is the thing affected by the action movere.

Here:

• puer = the boy
• saxum = the rock/stone

Since the boy is trying to move the rock, saxum is in the accusative case, which is the usual case for a direct object.

So:

• puer = nominative, subject
• saxum = accusative, object

Because after possum (I am able, I can) and its forms, Latin normally uses an infinitive.

So the structure is:

• potest = is able / can
• movere = to move

Together:

• movere potest = is able to move or can move

This is very common in Latin:

• currere potest = he can run
• videre possunt = they can see

Potest is the third person singular present active indicative of possum, posse, meaning to be able.

So potest means:

• he can
• she can
• it can
• or more literally, is able

Because the subject is puer (boy), here it means he can or, with non, he cannot.

Non simply negates the verb phrase.

So:

• potest = can
• non potest = cannot / is not able

In this sentence, non is understood with the whole idea movere potest:

• movere non potest = cannot move

Latin often places non right before the finite verb, but word order can vary.

Because Latin has no articles. There is no separate word for the or a/an.

So:

• puer can mean boy, a boy, or the boy
• saxum can mean rock, a rock, or the rock

You figure out which is best from the context or from the translation given.

Yes. Latin word order is much more flexible than English word order because the endings show each word’s function.

So all of these could express basically the same idea:

• Puer saxum movere non potest.
• Puer non potest saxum movere.
• Saxum puer movere non potest.
• Movere saxum puer non potest.

The exact emphasis can change, but the core meaning stays the same because:

• puer is still nominative
• saxum is still accusative
• movere is still the infinitive
• potest is still the main finite verb

Because the ending -re marks the infinitive in this verb.

So movere is a single Latin word meaning to move.

English usually uses two words:

• to move

Latin often uses one infinitive form:

• movere

So the idea of to is built into the infinitive form itself.

It comes from the verb moveo, movere, movi, motum, meaning move.

Here:

• moveo = I move
• movere = to move

This is a second-conjugation verb, which you can recognize from the long -e- in movere.

Because puer is one of the second-declension masculine nouns that end in -er in the nominative singular.

So although many masculine second-declension nouns look like:

• servus
• dominus

some look like:

• puer = boy
• ager = field

This is just a normal noun pattern you learn as part of the declension system.

For puer, the e stays in the stem:

• nominative: puer
• genitive: pueri

It is singular.

Saxum is the accusative singular (and also nominative singular) form of a neuter second-declension noun.

Its basic forms are:

• singular: saxum
• plural: saxa

So in this sentence it means a rock / the rock, not rocks.

Yes. Both are correct.

• movere potest = can move
• movere potest = is able to move

And with non:

• movere non potest = cannot move
• movere non potest = is not able to move

English often prefers can/cannot, but is able to is a more literal way to see how possum works.

The main finite verb is potest, not movere.

That is important because:

• potest carries the person and number: he/she/it can
• movere is just the infinitive: to move

So the sentence is built around:

• subject: puer
• main verb: non potest
• complementary infinitive: movere
• object of that infinitive: saxum

In other words, the structure is:

The boy + cannot + move the rock.