Onus servae grave est, sed plaustrum id facilius portat.

Here servae is most naturally genitive singular: the burden of the slave woman / slave girl.

That form servae can also be:

• dative singular
• nominative plural

So a learner often has to decide from context. In onus servae, the genitive makes the best sense: the slave’s burden.

A dative reading, a burden for the slave, is grammatically possible in isolation, but much less likely here.

Because grave agrees with onus.

• onus is neuter singular nominative
• the adjective gravis, grave must match it
• so the correct form is grave, the neuter nominative singular

This happens because grave is a predicate adjective with est:
Onus ... grave est = The burden ... is heavy

So Latin is not using the dictionary form gravis here; it is using the form that fits onus.

Onus is nominative singular, because it is the subject of est.

So the structure is:

• onus = subject
• grave = predicate adjective
• est = verb

In other words: The burden is heavy.

A useful extra point: onus is a third-declension neuter noun. Its full dictionary form is onus, oneris.

Id is a pronoun meaning it, and it refers back to onus.

Latin often uses a pronoun instead of repeating the noun, just as English does:

• first clause: onus
• second clause: id = it

Here id is the direct object of portat, so it is accusative singular neuter.

Because the pronoun must agree with the noun it refers to, and it refers to onus, not to serva.

• onus is neuter
• so the pronoun must be neuter
• therefore id

If Latin wanted to refer to the slave woman, then a feminine pronoun such as eam could be used. But here the thing being carried is the burden, so id is correct.

Facilius is the comparative adverb meaning more easily.

It comes from facilis, facile = easy.

In Latin:

• adjective: facilis = easy
• adverb: facile = easily
• comparative adverb: facilius = more easily

So facilius does not describe a noun. It describes how the cart carries the burden. It modifies portat.

The sentence does not state the comparison explicitly, but it is understood from the context.

Latin often leaves the standard of comparison unstated when it is obvious. Here the idea is something like:

• the burden is heavy for the slave woman
• but the cart carries it more easily

In other words, more easily than the slave woman does is implied.

Because facilius is an adverb, not an adjective.

Adjectives agree with nouns.
Adverbs modify verbs, adjectives, or other adverbs.

Here facilius modifies portat:

• plaustrum = subject
• id = object
• facilius = tells us how it carries
• portat = verb

So there is no agreement to worry about.

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

This sentence uses a very normal prose order:

• Onus servae grave est
• sed plaustrum id facilius portat

A few useful points:

• grave est is a common way to place a predicate adjective with est
• id comes before portat, which is very natural for an object pronoun
• the verb often comes near the end of the clause

Latin changes word order mainly for emphasis, style, or clarity, not because the grammar depends heavily on position.

Because Latin has no articles.

So Latin does not have separate words corresponding exactly to English the or a/an.

That means:

• onus can mean a burden or the burden
• servae can mean of a slave woman or of the slave woman

You decide from context which English article is best.

Sometimes Latin can omit a form of to be, especially in poetry, short expressions, or very compressed style. But in ordinary prose, est is often stated clearly, as it is here.

So Onus servae grave est is the straightforward full form.
Leaving out est would be possible in some contexts, but not necessary here.

Because its subject, plaustrum, is singular.

• plaustrum = the cart
• portat = it carries

So the verb is third person singular.

If the subject were plural, the verb would be plural too.