Cum sol oritur, lux per fenestram in cubiculum intrat.

Here cum means when. It introduces a subordinate clause of time:

cum sol oritur = when the sun rises

In other contexts, cum can also mean with, but not here. In this sentence it is a conjunction, not a preposition.

Oritur comes from orior, oriri, which is a deponent verb. Deponent verbs look passive in form, but they have an active meaning.

So:

• oritur looks passive
• but it means he/she/it rises or comes up

This is very common in Latin, and orior is one of the standard early examples.

Sol is nominative singular. It is the subject of oritur.

So in sol oritur, the sun is the thing doing the action of rising.

Latin does not have articles like English the or a/an.

So:

• sol can mean sun, the sun, or sometimes even a sun, depending on context
• lux can mean light or the light

You understand which one is meant from the sentence as a whole.

Lux is nominative singular. It is the subject of intrat.

So:

• lux = the thing entering
• intrat = enters

In other words, the light enters.

Fenestram is accusative singular because it follows the preposition per.

Per takes the accusative and usually means through.

So:

• per fenestram = through the window

This is a good phrase to memorize as a pattern:

• per
  • accusative

Yes, in can take either the accusative or the ablative, depending on meaning.

• in + accusative = movement into something
• in + ablative = position in or inside something

Here there is motion:

lux ... in cubiculum intrat = light enters into the bedroom

So cubiculum is accusative singular because the light is moving into the room.

Cubiculum usually means bedroom, sleeping room, or sometimes simply room/chamber, depending on context.

So in cubiculum intrat can be understood as enters the bedroom or enters into the room. If your translation says bedroom, that is a very natural meaning.

Latin word order is much more flexible than English word order because Latin uses endings to show each word’s role in the sentence.

English depends heavily on position:

• The light enters the room

Latin can move words around more freely:

• lux ... intrat
• in cubiculum
• per fenestram

The sentence still makes sense because the endings tell you what each word is doing.

That said, this sentence is actually quite natural Latin:

• time clause first: Cum sol oritur
• main subject next: lux
• then prepositional phrases
• verb at the end: intrat

Putting the verb near the end is very common in Latin.

Because it is a subordinate time clause: when the sun rises.

In English, we usually put a comma after an introductory clause:

• When the sun rises, the light enters...

Latin punctuation was not standardized in ancient texts the way it is in modern printed editions, but in modern textbooks and editions, a comma here is perfectly normal and helpful.

Both verbs are present tense because the sentence describes something happening generally or habitually:

• oritur = rises
• intrat = enters

This is similar to English:

• When the sun rises, light enters the room

It expresses a regular fact, not just one single event.

Yes. Cum is very common with both the indicative and the subjunctive, but the meaning changes.

Here, cum introduces a simple time clause:

• cum sol oritur = when the sun rises So the indicative is used.

But in other sentences, cum with the subjunctive can mean things like:

• when
• since
• although

often with a more explanatory or circumstantial sense.

So a learner should notice that cum does not always behave the same way; you have to look at the kind of clause it introduces.

They are:

• cum = conjunction meaning when
• sol, solis = sun
• orior, oriri, ortus sum = rise, arise
• lux, lucis = light
• per = through
• fenestra, fenestrae = window
• in = in, into
• cubiculum, cubiculi = bedroom, chamber
• intro, intrare, intravi, intratum = enter

Knowing the dictionary form helps you recognize the case endings and verb forms more easily.