Cum sol in caelo ascendit, puella fenestram aperit.

Cum can mean different things depending on how it is used.

• As a preposition with the ablative, it means with:
  cum amico = with a friend
• As a conjunction introducing a clause, it can mean when, since, or although.

In Cum sol in caelo ascendit, puella fenestram aperit, cum introduces a whole clause:

• cum sol in caelo ascendit = when the sun rises / as the sun rises

So here it is not a preposition at all; it is a conjunction.

Because this is a straightforward temporal cum clause: it simply tells you when something happens.

• cum
  • indicative often gives a plain time idea: when
• cum
  • subjunctive often gives a more contextual meaning such as when, since, or although in a more descriptive or explanatory way, especially in past narration

Here the sentence is simple and direct:

• Cum sol in caelo ascendit = When the sun rises

So the indicative ascendit is exactly what you would expect.

Both are present tense, third person singular.

• ascendit = he/she/it rises or is rising
• aperit = he/she/it opens or is opening

In this sentence:

• sol ... ascendit = the sun rises
• puella ... aperit = the girl opens

Latin present tense can be translated in more than one way in English depending on context, such as:

• rises
• is rising
• sometimes does rise

Yes, it could.

A Latin present-tense sentence like this can describe:

• a single event in a vivid way, or
• a repeated/habitual action

So depending on context, it might mean either:

• When the sun rises, the girl opens the window
• Whenever the sun rises, the girl opens the window

If the sentence is standing alone, many learners should understand it first as a simple general statement.

Because sol is the subject of ascendit.

In the clause sol in caelo ascendit:

• sol = the sun
• ascendit = rises

The thing doing the action is the subject, and in Latin the subject is usually in the nominative case.

So:

• sol is nominative singular

For the same reason: puella is the subject of the main verb aperit.

In puella fenestram aperit:

• puella = the girl
• aperit = opens

Since the girl is doing the action, puella is nominative singular.

Because it is the direct object of aperit.

In puella fenestram aperit:

• the girl is doing the opening
• the window is the thing being opened

In Latin, the direct object is commonly put in the accusative case.

So:

• fenestra = nominative form, window
• fenestram = accusative singular, window as object

Because in can take either the ablative or the accusative, and the choice depends on meaning.

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

Here the sentence says the sun rises in the sky, meaning its location is being described, not movement into the sky.

So:

• in caelo = in the sky
• in caelum would mean into the sky

That is why the ablative caelo is used.

Caelo is ablative singular.

It is ablative because it follows in in the sense of location:

• in caelo = in the sky

This is a very common Latin pattern:

• in urbe = in the city
• in aqua = in the water
• in horto = in the garden

So caelo is not the subject or object; it is part of a prepositional phrase showing where the action happens.

Latin word order is much more flexible than English word order because Latin uses case endings to show the job of a word in the sentence.

English depends heavily on position:

• The girl opens the window is not the same as The window opens the girl

Latin, however, marks roles with endings:

• puella = subject
• fenestram = object

So Latin can move words around more freely without causing confusion.

In this sentence, the verbs come at the end of their clauses:

• Cum sol in caelo ascendit
• puella fenestram aperit

That is a very common and natural Latin style, though other orders are possible too.

In modern printed Latin, a comma before or after a subordinate clause like this is normal and helpful:

• Cum sol in caelo ascendit, puella fenestram aperit.

But ancient Roman writing did not use punctuation the way modern texts do. So the comma is mainly a modern editorial aid.

It helps show that:

• cum sol in caelo ascendit is the subordinate clause
• puella fenestram aperit is the main clause

Classical Latin has no definite article and no indefinite article.

So a noun like sol can mean:

• sun
• the sun
• sometimes even a sun, depending on context

Likewise:

• puella can mean girl or the girl
• fenestram can mean a window or the window

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

A learner might look them up like this:

• cum — conjunction: when; also preposition with ablative: with
• sol, solis (m.) — sun
• in — preposition: in/on with ablative, into/onto with accusative
• caelum, caeli (n.) — sky, heaven
• ascendo, ascendere, ascendi, ascensum — to climb, rise, ascend
• puella, puellae (f.) — girl
• fenestra, fenestrae (f.) — window
• aperio, aperire, aperui, apertum — to open

From those forms you can identify:

• ascendit as from ascendere
• aperit as from aperire
• caelo as a form of caelum
• fenestram as a form of fenestra

There are two clauses, and each has its own subject and verb.

First clause:

• Cum sol in caelo ascendit
• subject: sol
• verb: ascendit

Second clause:

• puella fenestram aperit
• subject: puella
• verb: aperit
• object: fenestram

The structure is:

• When [the sun rises in the sky], [the girl opens the window].

Once you spot the verb endings and the noun cases, the sentence becomes much easier to untangle.