Feles in cellario manet, ut murem capiat.

Because feles, felis is a third-declension noun whose nominative singular happens to end in -es. In Latin, an ending like -es is often plural, but not always.

Here, feles is singular, and you can confirm that from the verb manet, which is also singular: the cat stays.

Manet is the 3rd person singular present active indicative of maneo, manere.

It means stays, remains, or waits depending on context. In this sentence, it describes what the cat is doing now.

Because in with the ablative usually means in or inside, showing location.

So:

• in cellario = in the cellar / in the pantry
• location, not movement

If the sentence meant into the cellar, Latin would normally use in with the accusative:

• in cellarium = into the cellar

Here ut introduces a purpose clause.

That means it gives the purpose of the main action:

• Feles in cellario manet = the cat stays in the cellar
• ut murem capiat = in order to catch the mouse

So here ut is best understood as in order that, so that, or more naturally in English, in order to.

Because after ut in a purpose clause, Latin normally uses the subjunctive.

So:

• capit = indicative, he/she catches
• capiat = subjunctive, he/she may catch or might catch

In this sentence, the cat has not necessarily caught the mouse yet. The clause expresses the cat’s purpose or intention, not a simple fact.

This is because of Latin sequence of tenses.

The main verb is manet, which is in a present tense. After a present main verb, a purpose clause usually uses the present subjunctive.

So:

• manet ... ut capiat = present main verb + present subjunctive

If the main verb were past, you would often expect the imperfect subjunctive instead:

• manebat ... ut caperet

Because murem is the accusative singular, and it is the direct object of capiat.

The dictionary form is:

• mus, muris = mouse

Its accusative singular is:

• murem = mouse, as the thing being caught

So:

• mus = a mouse as subject
• murem = a mouse as object

No. Classical Latin does not have definite and indefinite articles like English the and a.

So feles could mean:

• the cat
• a cat

And murem could mean:

• the mouse
• a mouse

You decide from context which sounds best in English.

Yes. Latin word order is much freer than English word order because the endings show the grammatical roles.

This sentence could be rearranged in several ways without changing the basic meaning, for example:

• In cellario feles manet, ut murem capiat.
• Feles ut murem capiat in cellario manet.

The original order is perfectly normal and clear. Different orders can change emphasis, but not necessarily the core meaning.

The comma is a matter of modern punctuation, used to make the sentence easier to read by marking off the subordinate clause.

Ancient Latin texts did not use punctuation the way modern editions do. So the comma is helpful, but it is not the thing that creates the grammar. The grammar comes from ut and the subjunctive capiat.

Yes, feles is usually treated as a feminine noun in Latin.

That does not necessarily mean the actual cat is female. Grammatical gender and biological sex are not always the same thing. In many sentences, feles can simply mean cat without stressing whether it is male or female.