Serva, immo ancilla nova, candelam fert et ostium claudit.

This is a correction or refinement.

• serva = female slave
• ancilla nova = new maidservant / new slave-girl

The word immo shows that the speaker is replacing the first description with a more precise one:

• Serva, immo ancilla nova...
• A slave woman—no, rather, a new maidservant...

So serva and ancilla nova refer to the same person.

Immo is a word used to correct, strengthen, or improve what was just said.

Common English equivalents are:

• no, rather
• indeed rather
• or better

So here it means something like:

• A slave woman—rather, a new maidservant—carries a candle and shuts the door.

It does not introduce a completely new idea; it revises the previous word.

They are all in the nominative singular feminine.

Why?

• serva is the subject
• ancilla nova is in apposition to serva, so it must be in the same case
• nova is an adjective modifying ancilla, so it agrees with it in:
  • gender: feminine
  • number: singular
  • case: nominative

So:

• serva = nominative singular
• ancilla = nominative singular
• nova = nominative singular feminine

Because Latin adjective position is more flexible than English.

Both of these are possible:

• ancilla nova
• nova ancilla

Both mean new maidservant.

In many basic Latin sentences, putting the adjective after the noun is very common and natural. English speakers often expect the adjective to come first, but Latin does not require that.

Because candelam is the direct object of fert.

• dictionary form: candela
• accusative singular: candelam

The verb fert means carries / bears / brings, and the thing being carried is the direct object, so Latin puts it in the accusative case.

So:

• candela = a candle, nominative
• candelam = a candle, accusative

Because ostium is a neuter noun.

In Latin, neuter nominative singular and neuter accusative singular are often identical.

So:

• ostium = door, doorway
• nominative singular: ostium
• accusative singular: ostium

Even though it is the direct object of claudit, its form does not change.

This is very common with neuter nouns.

Fert is:

• 3rd person singular
• present tense
• from the irregular verb ferre

So it means:

• he carries
• she carries
• it carries

In this sentence, because the subject is serva / ancilla nova, it means she carries.

A useful note: ferre is an important irregular verb. Its principal parts are:

• fero, ferre, tuli, latum

Claudit is:

• 3rd person singular
• present tense
• from claudere = to shut, close

The ending -t tells you the subject is he/she/it.

So claudit means:

• he shuts
• she shuts
• it shuts

Here it means she shuts.

Latin usually does not need to state subject pronouns like he, she, or it, because the verb ending already shows the person and number.

• fert = she/he/it carries
• claudit = she/he/it shuts

And since the subject noun serva ... ancilla nova is already present, there is no need to add a separate word for she.

Latin can add a pronoun for emphasis, but it usually leaves it out.

Because Classical Latin has no articles.

English distinguishes:

• a candle
• the candle

Latin simply has:

• candelam

Likewise:

• ostium can mean a door or the door
• serva can mean a female slave or the female slave

You figure out the exact sense from the context.

Latin word order is flexible, because the endings show the grammatical relationships.

This sentence is basically:

• subject: Serva, immo ancilla nova
• object 1: candelam
• verb 1: fert
• et = and
• object 2: ostium
• verb 2: claudit

A very literal layout would be:

• The slave woman—rather, the new maidservant—candle carries and door shuts.

That sounds odd in English, but it is normal Latin structure.

Because et is simply linking the two actions:

• fert
• claudit

So the sentence says that the same subject does two things:

1. candelam fert
2. ostium claudit

Latin often uses et exactly this way, just like English and.

Yes, serva can also be the singular imperative of servare, meaning save! or keep!

But here it is clearly a noun, not a verb.

Why?

• it is followed by immo ancilla nova, which explains or corrects it
• the sentence already has its finite verbs: fert and claudit
• the structure makes serva the subject

So in this sentence, serva definitely means female slave.

They are close in meaning, but not always exactly identical in tone.

• serva is the straightforward feminine form of servus: female slave
• ancilla often means maidservant, slave-girl, or female attendant

In many contexts they overlap a lot. Here, because of immo, the speaker seems to think ancilla nova is a better or more precise label than serva.

So the difference is not mainly grammar; it is a matter of word choice and nuance.