Puer cum amico suo laetior est quam solus.

Puer is the subject of the sentence, so it is in the nominative singular.

• puer = boy
• nominative is the case usually used for the subject
• the verb est is singular, so the subject is singular too

So Puer ... est means The boy ... is.

The preposition cum means with, and it takes the ablative case.

So:

• amicus = friend (dictionary form, nominative)
• amico = with a friend / with the friend (ablative singular after cum)

This is a very common Latin pattern:

• cum amico = with a friend
• cum puero = with the boy
• cum familia = with the family

Suo is the possessive adjective his/her/its own, and here it agrees with amico.

That means:

• amico is masculine singular ablative
• suo is also masculine singular ablative

So cum amico suo means with his own friend or more naturally in English with his friend.

A key point: suus, sua, suum usually refers back to the subject of the clause.
So here it refers back to puer.

Latin often uses suus when the possessor is the subject of the sentence.

Here, the subject is puer, so:

• cum amico suo = with his own friend / with his friend = the boy’s friend

If Latin used eius, that would usually mean his in the sense of someone else’s, not the subject’s own.

So the distinction is roughly:

• suus = belonging to the subject
• eius = belonging to another person already mentioned or understood

Laetior is the comparative form of laetus, which means happy.

So:

• laetus = happy
• laetior = happier

The ending -ior is a normal way to form the comparative of many Latin adjectives.

Examples:

• altus = high

• altior = higher

• fortis = brave
• fortior = braver

So laetior est means is happier.

Laetior means happier, but Latin still needs est to say is happier.

So:

• laetior = happier
• laetior est = is happier

This works like English:

• happy
• is happy
• happier
• is happier

Quam means than in a comparison.

So:

• laetior est quam solus = he is happier than alone

In smoother English, we would usually say:

• The boy is happier with his friend than when he is alone
• or The boy is happier with his friend than alone

So quam is the word that marks the comparison.

This is a very common question.

Solus is nominative masculine singular, because it refers back to puer. It describes the boy, not the friend.

So the idea is:

• Puer ... laetior est quam solus
• literally: The boy is happier than alone
• more fully: The boy is happier than he is when he is alone

Because solus describes puer, it matches puer in case, gender, and number:

• puer = nominative masculine singular
• solus = nominative masculine singular

If it were solo, that would be ablative, and that would not fit this structure.

Yes. Solus is an adjective meaning alone or by oneself.

Even though English often treats alone almost like an adverb, Latin uses an adjective here. It agrees with the person being described.

So solus means:

• alone (masculine singular)
• describing puer

If the subject were feminine, you would expect sola.
If plural masculine, soli.

In a sense, yes. Latin often leaves out words that are easy to understand from context.

The full idea is something like:

• Puer cum amico suo laetior est quam solus est.

But Latin does not need to repeat the second est, because it is obvious.

So quam solus is a shortened comparison:

• than when he is alone
• than he is alone

Yes, Latin word order is much freer than English word order.

This sentence puts the words in a perfectly normal order:

• Puer = subject first
• cum amico suo = prepositional phrase
• laetior est = predicate
• quam solus = comparison

But Latin could rearrange this for emphasis. For example, the following would still mean essentially the same thing:

• Cum amico suo puer laetior est quam solus.
• Puer laetior est cum amico suo quam solus.

The endings show the grammar, so the order can move more easily than in English.

Not necessarily. Suus can sometimes be translated emphatically as his own, but often in ordinary sentences it simply means his, as long as it refers back to the subject.

So here, depending on context, you could understand it as:

• with his friend
• or, if you want to show the Latin more closely, with his own friend

Usually English would just say with his friend unless there is special emphasis.

Yes. Latin sometimes uses the ablative of comparison instead of quam with a comparative adjective.

But in this sentence, quam + solus is the natural structure because solus is an adjective referring back to the subject, and the comparison is really a shortened clause: than when he is alone.

So quam works very well here.

A helpful way to break it down is this:

• Puer = subject
• cum amico suo = with his friend
• laetior est = is happier
• quam solus = than alone / than when he is alone

So the sentence is built like this:

The boy + with his friend + is happier + than alone.

That is a very typical Latin pattern:

• subject
• extra phrase
• predicate with sum
• comparison with quam