Nocte obscura feles murem in cellario exspectat.

Because this is an example of the ablative of time when. Latin often uses the ablative without a preposition to say when something happens.

• nocte = ablative singular of nox, noctis = night
• obscura = ablative singular feminine of obscurus, -a, -um = dark

So nocte obscura means on a dark night or at night, when it is dark.

Latin often does not need a preposition for expressions of time. English says at night or on a dark night, but Latin can simply use the ablative case by itself.

So instead of something like in nocte obscura, classical Latin very naturally says nocte obscura.

Because feles is in the nominative, the case typically used for the subject of the sentence.

Also, the verb exspectat is third person singular, so it matches a singular subject: the cat waits.

So:

• feles = subject = the cat
• exspectat = waits / is waiting for

Feles is a third-declension noun, not a first- or second-declension one. Its dictionary form is usually given as:

• feles, felis

That is why it does not end in -a or -us in the nominative singular.

A learner coming from early Latin often expects subjects to look like puella or servus, but many common Latin nouns belong to the third declension and have more varied forms.

Because murem is the direct object of exspectat.

The verb exspectare means to wait for, to expect, or to await, and the thing being awaited goes into the accusative.

• mus, muris = mouse
• murem = accusative singular = the mouse

So:

• feles = the cat
• murem = the mouse that the cat is waiting for

Exspectat is:

• third person singular
• present tense
• active voice
• indicative mood

from the verb exspectare.

So it means he/she/it waits for, is waiting for, or expects, depending on context. Here it means the cat waits for.

Because in takes different cases depending on meaning:

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

Here the cat is already located in the cellar, so Latin uses the ablative:

• in cellario = in the cellar

If the sentence meant the cat goes into the cellar, then you would expect in cellarium.

Yes. Adjectives in Latin agree with the nouns they describe in:

• gender
• number
• case

Here:

• nocte is feminine, singular, ablative
• obscura is also feminine, singular, ablative

That agreement shows that obscura describes nocte.

Latin word order is much freer than English word order because the case endings show what each word is doing.

Putting the verb at the end is very common in Latin prose. This sentence follows a very natural Latin pattern:

• time expression first: Nocte obscura
• subject: feles
• object: murem
• place phrase: in cellario
• verb last: exspectat

English relies much more on word order, but Latin relies much more on endings.

Yes, often they could. For example, Latin could say:

• Feles nocte obscura murem in cellario exspectat
• Murem feles in cellario nocte obscura exspectat

The basic meaning would stay the same because the endings still mark the roles of the words. However, changing the order changes the emphasis.

In the given sentence, Nocte obscura comes first, which sets the scene dramatically: On a dark night...

Latin has no definite or indefinite article. It does not have separate words for the or a/an.

So:

• feles can mean a cat or the cat
• murem can mean a mouse or the mouse
• cellario can mean a cellar or the cellar

You choose the best English article from the context.

Not necessarily in translation. The noun feles is grammatically feminine in most dictionaries and textbooks, but grammatical gender does not always force a natural-sex translation in English.

So English will usually just say the cat, unless the context clearly requires she-cat or female cat.

Yes. Spectat means looks at or watches. Exspectat means waits for, expects, or awaits.

So the sentence is not saying the cat is merely looking at the mouse. It is saying the cat is waiting for the mouse, probably lying in wait.

That makes the sentence feel more purposeful and vivid.