Serva in vestibulo saccum gravem ponit, sed puella saccum levem ad culinam fert.

Both serva and puella are nominative singular forms, so they are the subjects of their verbs.

• serva = the slave woman / female servant
• puella = the girl

In this sentence:

• Serva ... ponit = The slave woman ... places
• puella ... fert = the girl ... carries

The -a ending here is a very common nominative singular ending for first-declension feminine nouns.

Saccum is in the accusative singular, which marks it as the direct object.

It is the thing being acted on:

• the slave woman places the sack
• the girl carries the sack

The basic dictionary form is saccus. In the accusative singular, a second-declension masculine noun like this usually changes:

• saccus → saccum

So saccum means the sack as the object of ponit or fert.

Because adjectives in Latin must agree with the nouns they describe in:

• gender
• number
• case

Here, both adjectives describe saccum, so they must match it.

Since saccum is:

• masculine
• singular
• accusative

the adjectives must also be masculine singular accusative:

• gravis → gravem
• levis → levem

So:

• saccum gravem = the heavy sack
• saccum levem = the light sack

These use two different prepositions with two different ideas:

• in + ablative = in / on for location
• ad + accusative = to / toward for motion toward

So:

• in vestibulo = in the entrance hall or in the foyer
  This tells you where the sack is placed.
• ad culinam = to the kitchen
  This tells you where the girl is carrying it to.

This is a very important Latin pattern:

• in with the ablative often means where
• ad with the accusative often means to where

Because vestibulo is ablative singular, used after in when in means location.

The noun’s basic form is vestibulum, but after in for location it becomes:

• vestibulum → vestibulo

So:

• in vestibulo = in the entrance hall

If Latin wanted to express movement into the entrance hall, it could use in with the accusative instead.

Because ad takes the accusative.

The basic form is culina, but after ad it becomes:

• culina → culinam

So:

• ad culinam = to the kitchen

This is another very standard Latin construction:

• ad + accusative = motion toward a place

In Latin, the endings usually tell you the job of each word more than the word order does.

Here:

• serva = nominative singular → subject
• puella = nominative singular → subject
• saccum = accusative singular → direct object

So even if the order changes, the endings still show the roles.

For example, Latin could rearrange the words in several ways and still mean basically the same thing, because:

• serva is still the subject
• saccum is still the object

English depends much more on word order; Latin depends much more on forms.

Latin adjective position is more flexible than English adjective position.

English normally says:

• the heavy sack
• the light sack

Latin can say:

• saccum gravem
• gravem saccum

Both are possible. In this sentence, the adjective comes after the noun, which is very common in Latin.

So you should not assume that a Latin adjective must always come before the noun the way it often does in English.

They are both third-person singular present tense verbs, but they mean different kinds of actions.

• ponit = puts, places, sets down
• fert = carries, brings

So:

• Serva ... saccum gravem ponit = the slave woman places the heavy sack
• puella ... saccum levem ad culinam fert = the girl carries the light sack to the kitchen

The first action emphasizes putting something somewhere.
The second emphasizes transporting something.

Because they come from different verbs with different stems:

• pono, ponere → ponit
• fero, ferre → fert

Even though both are third-person singular present, different verbs can form that tense differently.

Still, both mean he/she/it ...s in English:

• ponit = she puts
• fert = she carries

So the matching idea is grammatical person and tense, not identical spelling.

Sed means but.

It connects the two clauses and shows a contrast:

• the slave woman places a heavy sack in the entrance hall,
• but the girl carries a light sack to the kitchen.

So sed signals that the second clause is being set against the first one in some way.

Yes. Their dictionary forms are:

• gravis, grave = heavy
• levis, leve = light

These are third-declension adjectives, so their endings do not look like first/second-declension adjectives such as bonus, bona, bonum.

That is why the accusative singular masculine/feminine form is:

• gravem
• levem

rather than something like -um.

Latin often does repeat nouns for clarity, even when English might avoid repetition.

Here we get:

• saccum gravem
• saccum levem

That makes the contrast very clear:

• one sack is heavy
• the other sack is light

Latin could sometimes omit repeated words if the meaning were obvious, but repeating saccum here is perfectly natural and helps emphasize the contrast.

A more literal version would be something like:

The slave woman in the entrance hall places a heavy sack, but the girl a light sack to the kitchen carries.

That sounds awkward in English, but it helps show the Latin structure.

A normal English translation would be:

The slave woman puts the heavy sack in the entrance hall, but the girl carries the light sack to the kitchen.

This is a good reminder that you should translate by sense, not just by copying Latin word order.

Grammatically and logically, in vestibulo goes with the action ponit.

It tells you where the placing happens or where the sack is placed:

• Serva in vestibulo saccum gravem ponit
  = The slave woman puts the heavy sack in the entrance hall

It is not mainly describing the slave woman herself as if it meant the slave woman who is in the entrance hall, although context could sometimes blur those ideas. In ordinary reading, it is taken with the verb.

They are both present tense.

More specifically, they are:

• third person
• singular
• present active indicative

So they mean:

• she places / puts
• she carries

Because the subjects are serva and puella, English uses she.

Classical Latin has no articles like English the or a/an.

So a noun like serva can mean, depending on context:

• a slave woman
• the slave woman

Likewise saccum can mean:

• a sack
• the sack

English translators supply a or the based on context and natural style. Latin simply does not mark that distinction with a separate word the way English does.