Learn Hardcore Latin: Dominus in sella sedet, servus ante eum stat. - The master sits in the chair, the servant stands before him.

They are both in the nominative case (the case normally used for the subject).
Their endings -us are the typical nominative singular endings for masculine nouns of the second declension.

Why do dominus and servus both end in -us? What does that ending mean?

The ending -us here marks:

Case: nominative (subject form)
Number: singular
Gender: masculine
Declension: second declension

So dominus = master (as subject), servus = slave/servant (as subject).

If these words were direct objects, you would normally see -um instead:

dominum = master (object)
servum = slave/servant (object)

What tense and person are sedet and stat, and how do they relate to English?

Both sedet and stat are:

Person: 3rd person (he/she/it)
Number: singular
Tense: present
Mood/voice: indicative active

So:

sedet = he/she/it sits (or is sitting)
stat = he/she/it stands (or is standing)

The infinitives are:

sedēre = to sit
stare = to stand

Why is it in sella, not something like in sellam?

Latin uses different cases with in depending on the meaning:

in
- ablative = in / on (location, “where?”)
in
- accusative = into / onto (motion, “where to?”)

Here in sella means on the chair / in the chair (a place where someone is), so sella is in the ablative singular form.

If the meaning were onto the chair / into the chair, then you would expect in sellam (accusative).

What form is sella here, and what does it tell me?

sella is:

First declension
Feminine
Singular
Ablative case (because of in with the meaning “in/on”)

So in sella literally means in/on (a/the) chair.

The main forms of sella (chair) are:

Nominative: sella (chair as subject)
Accusative: sellam (chair as object)
Ablative: sellā (often written without a macron; here it’s the same spelling sella in most textbooks, but pronounced differently)

Why is it ante eum and not something like ante dominum or ante domino?

Two points:

ante always takes the accusative case, so it must be followed by an accusative form:
- eum (accusative singular of is, “he”) = “him”
- dominum would also be accusative, and would be grammatically correct: servus ante dominum stat = the slave stands before the master.
The sentence chooses eum instead of repeating dominus to avoid repetition. eum refers back to dominus: the slave stands in front of him = in front of the master.

ante domino would be wrong because domino is dative/ablative, and ante does not take those cases.

What exactly does eum mean, and how do I know what it refers to?

eum is:

The accusative singular masculine form of the pronoun is, ea, id.
It means him (or it, if referring to a masculine noun).

In this sentence, eum can only reasonably refer back to dominus, the previously mentioned masculine noun in the same clause. So ante eum = in front of him = in front of the master.

Latin often uses pronouns like this instead of repeating the noun again.

Latin doesn’t have “the” or “a” in this sentence. How do I know if it’s the master or a master?

Latin has no separate words for “the” or “a/an”. The bare noun dominus can mean:

the master
a master

Which article you use in English depends on context and style. In a simple example sentence like this, English usually prefers the master and the slave because we are talking about particular people in a scene.

Can the word order be changed? For example, could I say Dominus sedet in sella, servus stat ante eum?

Yes. Latin word order is much more flexible than English.

Dominus in sella sedet and Dominus sedet in sella both mean the same thing.
servus ante eum stat and ante eum servus stat also mean the same thing.

The endings (cases and verb endings) show who is doing what to whom, so word order is often used for emphasis or style, not for basic grammar. The original order is very natural, but your alternatives are also correct.

Could I leave out the nouns and just say In sella sedet, ante eum stat?

Yes, grammatically you can, because Latin is a pro-drop language: the verb ending already tells you the subject is he/she/it.

So:

in sella sedet = he/she sits on a/the chair
ante eum stat = he/she stands in front of him

However, without dominus and servus, you would not know from this sentence alone who is sitting and who is standing. In a longer context, if it was already clear who is doing what, Latin might omit the explicit nouns.

If servus is masculine, how would I say “female slave/maid” in a similar sentence?

The feminine counterpart to servus is serva.

For example:

Domina in sella sedet, serva ante eam stat.
- domina = mistress (female master)
- serva = female slave/maid
- eam = her (feminine accusative, referring to domina)

This is the same structure as the original sentence, but everything is in the feminine.