Cum nox obscura esset, pater lucernam accendit.

Here cum introduces a subordinate clause: cum nox obscura esset.

In this kind of sentence, cum often means when, while, or since, depending on context. In this sentence, it is best understood as when or since/because the night was dark.

So the clause gives the circumstance/background for the main action:

• cum nox obscura esset = when the night was dark / since the night was dark

This is a very common use of cum in Latin narrative.

This is one of the most common questions learners ask.

Latin often uses cum + subjunctive for a background or circumstantial clause in past-time narration. That is exactly what is happening here:

• esset = imperfect subjunctive of esse
• erat = imperfect indicative of esse

So:

• cum nox obscura erat would be a more straightforward when the night was dark
• cum nox obscura esset gives the clause a more literary, background-setting feel: when/since the night was dark

In other words, the subjunctive here is not mainly about doubt. It is used because Latin regularly puts cum clauses of circumstance in the subjunctive.

Esset is imperfect subjunctive active, from sum, esse.

Its job here is to describe an ongoing background situation in the past:

• nox obscura esset = the night was dark

The imperfect is appropriate because the darkness is not a single momentary event; it is the condition in the background while the father performs the main action.

Because nox is the subject of the cum clause, it must be in the nominative singular.

• nox = night (nominative singular)
• obscura = dark (feminine nominative singular adjective)

The adjective must agree with the noun in gender, number, and case, so:

• nox is feminine singular nominative
• therefore obscura is also feminine singular nominative

That is why the phrase is nox obscura.

Because it describes nox, and nox is a feminine noun.

In Latin, adjectives agree with the nouns they describe in:

• gender
• number
• case

So:

• nox = feminine singular nominative
• obscura must match it

Even though English adjectives do not change form, Latin adjectives do.

Pater is nominative singular, because it is the subject of the main verb accendit.

So the basic structure is:

• pater = the father
• accendit = lit / lights

That makes pater the one doing the action.

Lucernam is accusative singular because it is the direct object of accendit.

The father is doing the action of lighting, and the thing being lit is the lamp:

• pater = subject
• lucernam = direct object

Dictionary form:

• lucerna, -ae = lamp

So:

• lucerna = nominative singular
• lucernam = accusative singular

Formally, accendit can be either:

• present: he lights
• perfect: he lit / has lit

In this sentence, because the clause before it is past background narration (cum nox obscura esset), accendit is most naturally understood as perfect:

• the father lit a lamp

This is very common in Latin: the verb form accendit can be ambiguous by itself, and the context tells you which tense is meant.

Latin has no definite article like English the, and no indefinite article like a/an.

So:

• nox can mean night or the night
• pater can mean father or the father
• lucernam can mean a lamp or the lamp

You decide from context which English article makes the best translation.

Yes, the words could be arranged differently.

Latin word order is more flexible than English because the endings show each word’s role. This sentence is arranged in a very natural and clear way:

• Cum nox obscura esset, pater lucernam accendit.

But Latin could also say things like:

• Pater, cum nox obscura esset, lucernam accendit.
• Lucernam pater accendit, cum nox obscura esset.

The meaning would stay basically the same, though the emphasis might change.

This sentence places the cum clause first to set the scene before the main action.

Yes, that is possible, but it may sound less natural in English.

Latin cum often gives a background circumstance, so translations such as these can all work depending on context:

• when the night was dark
• since the night was dark
• because the night was dark
• while it was dark at night would be less direct
• while the night was dark is grammatical, but a little awkward in normal English

Usually the smoothest translation is something like When the night was dark, the father lit a lamp or Since the night was dark, the father lit a lamp.

Because that is not what the sentence is saying.

The clause does not simply say at night. It says:

• the night was dark

So nox is the subject of the verb esset, not the object of a preposition.

If Latin wanted to say something more like at night, it might use a different construction. But here the idea is specifically that the darkness of the night is the circumstance that leads to the father lighting the lamp.

Here is the structure:

• Cum = introduces a subordinate clause
• nox = subject of that clause
• obscura = adjective describing nox
• esset = imperfect subjunctive verb of the cum clause
• pater = subject of the main clause
• lucernam = direct object of the main clause
• accendit = main verb

So the sentence has:

1. a background subordinate clause
   
   • cum nox obscura esset
2. a main clause
   
   • pater lucernam accendit

That is a very common Latin sentence pattern.