Learn Hardcore Latin: Puer solus in cubiculo sedet, quamquam soror nostra foris ridet. - The boy sits alone in the bedroom, although our sister is laughing outside.

puer solus usually emphasizes that the boy is alone (no one is with him).
solus puer tends more to mean “only the boy” (no one else, only he).

So:

puer solus in cubiculo sedet – the boy is alone in the room (no company).
solus puer in cubiculo sedet – only the boy is in the room (others are not there), though he might or might not feel “alone.”

In your sentence, puer solus stresses his being alone, not the idea of “only the boy.”

What case is in cubiculo, and why is it in that case?

Cubiculo is ablative singular (from cubiculum, cubiculi – room/bedroom).

The preposition in with the ablative expresses location (“in, on, at” where something is):

in cubiculo = “in the room” / “in the bedroom.”

If the verb expressed motion toward the room (going into it), in would take the accusative:

in cubiculum intrat – “He enters into the room.”

But sedet (“sits”) is a state, not motion, so in + ablative is used.

What exactly is the form of sedet, and what tense is it?

Sedet is:

3rd person singular
Present tense
Indicative mood
Active voice

From the verb sedeo, sedere, sedi, sessum – “to sit.”

So puer … sedet means:

“the boy sits” or
“the boy is sitting.”

In Latin, the present tense can correspond to either simple present (sits) or present continuous (is sitting) in English, depending on context.

What does quamquam mean, and what kind of clause does it introduce?

Quamquam means “although”.

It introduces a concessive subordinate clause – one that presents something that contrasts with, but does not prevent, the action of the main clause.

Main clause: Puer solus in cubiculo sedet – “The boy sits alone in the room.”
Concessive clause: quamquam soror nostra foris ridet – “although our sister is laughing outside.”

Classically, quamquam is usually followed by the indicative (as here: ridet, present indicative), not the subjunctive.

Why is it soror nostra, and what is the function of nostra?

Soror (“sister”) is nominative singular feminine and is the subject of the verb ridet in the subordinate clause.

Nostra is a possessive adjective meaning “our.” It agrees with soror in:

Gender: feminine
Number: singular
Case: nominative

So:

soror nostra = “our sister.”

You could also say nostra soror; both orders are possible. The meaning is the same, though Latin often places possessive adjectives after the noun, especially in simple sentences like this.

Why is it soror nostra instead of something like nostrae sororis?

Latin normally expresses possession with possessive adjectives (like noster, nostra, nostrum) rather than with a genitive in many everyday contexts.

soror nostra – “our sister” (very natural and common)
nostrae sororis soror would literally be “the sister of our sister,” which is a different meaning.

A genitive like nostrae sororis would mean “of our sister,” not “our sister.” For basic possession (my/your/our/etc.), Latin usually prefers:

meus, mea, meum – my
tuus, tua, tuum – your (singular)
noster, nostra, nostrum – our
vester, vestra, vestrum – your (plural)

What is foris, and why isn’t it foras?

In your sentence, foris is an adverb meaning “outside” (locative sense: where?).

soror nostra foris ridet – “our sister is laughing outside.”

Classical Latin also has foras, another adverb, which usually means “out, to the outside” (direction: to where?).

So the difference is:

foris – “outside” (location; where something is)
foras – “out/outside” (motion toward; where something is going)

Here, ridet describes an action already taking place outside, not movement toward the outside, so foris is appropriate.

What is the form of ridet, and how should it be translated?

Ridet is:

3rd person singular
Present tense
Indicative mood
Active voice

From rideo, ridere, risi, risum – “to laugh” or “to smile (broadly).”

It can be translated as:

“she laughs”
“she is laughing”

So soror nostra foris ridet can mean:

“our sister is laughing outside” or
“our sister laughs outside.”

Context in English usually decides whether to use simple present or continuous.

Why does the Latin sentence have no word for “the” or “a”?

Latin does not have articles (no equivalent of English “the” or “a/an”).

The bare noun puer can mean:

“boy,”
“a boy,” or
“the boy,”

depending on context. Similarly, soror can mean “sister / a sister / the sister.”

Translators choose “a” or “the” in English based on what makes sense in the context. Here, “the boy” and “our sister” sound natural in English, even though Latin just has puer and soror nostra.

Could the word order be different, and would the meaning change?

The word order could be rearranged in several ways without changing the basic meaning, because Latin relies chiefly on endings, not word order, to show grammatical roles.

For example:

Solus puer in cubiculo sedet, quamquam nostra soror foris ridet.
In cubiculo puer solus sedet, quamquam foris soror nostra ridet.

The core grammatical relations stay the same:

puer is still the subject of sedet,
soror nostra is still the subject of ridet,
in cubiculo is still a location,
foris is still “outside,”
solus still describes the boy.

Different orders can add slight emphasis (for instance, putting solus first might highlight “alone”), but the fundamental meaning is unchanged.

Why is there no explicit pronoun like “he” or “she” in the Latin sentence?

Latin usually omits subject pronouns (like ego, tu, is, ea) because the verb ending tells you the person and number.

sedet = “he/she/it sits”
ridet = “he/she/it laughs”

To clarify who is doing the action, Latin often uses a noun instead of a pronoun:

puer … sedet – the boy sits (he sits)
soror nostra … ridet – our sister laughs (she laughs)

You would typically use explicit pronouns (is, ea, id, etc.) only for emphasis or contrast, or when you have no clear noun referring to the subject.