Puer manus frigidas fricat, quia per fenestram apertam aer frigidus intrat.

Puer is nominative singular, which marks it as the subject of fricat.

• puer = the boy
• fricat = rubs

So puer fricat means the boy rubs.

A native English speaker often expects word order to show the subject, but in Latin the case ending usually does that job.

It is the accusative plural of manus, a 4th-declension noun.

A few important points:

• manus can be:
  • nominative singular = hand
  • accusative plural = hands
• In many textbooks, the accusative plural is written manūs with a macron to show the long vowel.
• If macrons are omitted, both forms appear simply as manus.

So here manus frigidas means cold hands, and the adjective frigidas shows clearly that manus is plural accusative.

In Latin, grammatical gender does not always match natural expectations. Manus is simply a feminine noun.

So its adjective must also be feminine:

• manus frigidas = cold hands
• not manus frigidos

This is something you mostly just learn with the noun:

• manus, manūs, f. = hand

Because adjectives must agree with the nouns they describe in gender, number, and case.

Here:

• manus frigidas

  • manus = accusative plural, feminine
  • frigidas = accusative plural, feminine

• aer frigidus

  • aer = nominative singular, masculine
  • frigidus = nominative singular, masculine

So even though both words come from the idea cold, they take different endings because they describe different nouns.

Because it is the direct object of fricat.

The verb fricare means to rub, and in this sentence the boy is rubbing his hands. The thing being rubbed is the direct object, so Latin puts it in the accusative.

Structure:

• puer = subject, nominative
• manus frigidas = direct object, accusative
• fricat = verb

Because aer frigidus is the subject of intrat.

In the clause:

• aer frigidus intrat = cold air enters

The air is the thing doing the entering, so it is in the nominative.

An English speaker might initially think air is the object of through the open window, but it is not. The prepositional phrase per fenestram apertam just tells you where/through what the air enters.

Because the preposition per takes the accusative.

• per = through
• fenestram = accusative singular of fenestra
• apertam agrees with fenestram

So:

• per fenestram apertam = through the open window

This is a very common pattern in Latin:

• per + accusative

Because apertam describes fenestram.

It means open here, and it matches fenestram in:

• gender: feminine
• number: singular
• case: accusative

So:

• fenestra aperta = an open window / the open window (nominative)
• fenestram apertam = an open window / the open window (accusative)

Yes, historically it is the perfect passive participle of aperire (to open), so its basic sense is opened.

But in ordinary Latin it is also very commonly used like a normal adjective meaning open.

So in this sentence:

• fenestra aperta = an open window

English does the same kind of thing sometimes:

• a closed door
• a broken cup

So apertam can be understood naturally here as open.

Latin word order is much more flexible than English word order because the endings show the grammatical relationships.

This order emphasizes the route first:

• quia per fenestram apertam aer frigidus intrat
• literally, something like: because through the open window cold air enters

That sounds a bit unusual in English, but it is perfectly natural in Latin.

Latin often places:

• important setting information first,
• and the verb at or near the end.

Quia introduces a clause giving a reason:

• quia = because

So the sentence is divided like this:

• Puer manus frigidas fricat = main clause
• quia per fenestram apertam aer frigidus intrat = reason clause

In other words: the boy rubs his cold hands because cold air is coming in through the open window.

Because the sentence describes what is happening now or what happens in a vivid present scene.

• fricat = he rubs / is rubbing
• intrat = enters / is entering

Latin present tense can often correspond to either:

• simple present in English (rubs, enters), or
• progressive present (is rubbing, is entering),

depending on context.

Classical Latin normally has no articles.

So:

• puer can mean the boy or a boy
• fenestram apertam can mean the open window or an open window
• aer frigidus can mean the cold air or cold air

The context tells you which English article makes the best sense.

Not in this sentence, because frigidas shows that it is plural.

If it were singular, you would expect:

• manum frigidam = a cold hand

But here we have:

• manus frigidas = cold hands

So the adjective removes the ambiguity.

Latin often leaves possessive words unexpressed when they are obvious from context.

Here, if the boy rubs hands, the natural understanding is that he rubs his own hands.

Latin could say manus suas if it wanted to emphasize his own hands, but it does not need to.

So:

• manus frigidas fricat naturally means he rubs his cold hands

A learner might want the basic forms like these:

• puer, puerī = boy
• manus, manūs = hand
• frigidus, -a, -um = cold
• fricō, fricāre = rub
• quia = because
• per = through
• fenestra, fenestrae = window
• apertus, -a, -um (from aperiō, aperīre) = open, opened
• aer, aëris = air
• intrō, intrāre = enter

This is often the most useful way to connect the sentence to vocabulary study.