Saccus tam gravis est ut serva eum tollere non possit.

Tam ... ut is a very common Latin pattern meaning so ... that.

• tam = so
• ut = that

So in this sentence:

• tam gravis = so heavy
• ut serva eum tollere non possit = that the slave-girl cannot lift it

English uses the same basic idea, but in Latin this pattern is especially important because it usually introduces a result clause.

Because after tam ... ut, Latin normally uses a result clause, and result clauses take the subjunctive.

So:

• non potest would simply mean cannot
• non possit here means may be unable / cannot inside the result clause

In other words, possit is not there because the action is doubtful. It is there because Latin grammar requires the subjunctive after this kind of ut clause.

It is a result clause.

A result clause expresses what happens as a consequence of something. Here, the consequence of the sack being so heavy is that the slave-girl cannot lift it.

A good way to spot it is:

• tam in the main clause
• ut
  • subjunctive in the dependent clause

That combination strongly suggests result.

Because it depends on possit.

Latin uses an infinitive after verbs meaning be able, want, dare, begin, and similar ideas. So:

• possit tollere = is able to lift / can lift

Here:

• non possit tollere = cannot lift

This is very similar to English can lift, except Latin uses a form of posse plus the infinitive.

Because eum is the accusative singular masculine form of is, ea, id, meaning him / it depending on context.

Since saccus is a masculine noun, the pronoun referring back to it must also be masculine:

• saccus = masculine
• eum = masculine accusative singular

It is accusative because it is the direct object of tollere.

So eum tollere means to lift it.

The dictionary form tells you that. Saccus is a second-declension masculine noun.

Also, the pronoun eum confirms this, because it agrees with saccus in gender.

A learner might expect a thing to be neuter, but Latin nouns do not work that way. Grammatical gender is something you memorize with the noun.

Because gravis must agree with saccus.

• saccus is masculine singular nominative
• so the adjective must also be masculine singular nominative
• that form is gravis

If the noun were neuter, you might see grave instead.

So saccus gravis means a heavy sack or the sack is heavy.

Serva is nominative singular.

It is the subject of possit, so it means the slave-girl is the one who is unable to do the lifting.

That can be confusing because the main subject of the whole sentence is saccus, but inside the ut clause there is a different subject:

• main clause subject: saccus
• result clause subject: serva

So the sentence has two clauses, each with its own subject.

More specifically, serva means female slave or slave-woman.

Compare:

• servus = male slave
• serva = female slave

A native English speaker might miss this because English usually does not mark gender in the noun itself.

Because non normally negates the verb.

So:

• possit = may be able / can
• non possit = cannot

Latin word order is flexible, but non is very often placed directly before the word it negates, especially a verb.

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

For example, the sentence could be rearranged in several ways and still mean the same thing, as long as the forms stay the same. But the given order is natural and clear:

• Saccus tam gravis est gives the main statement first
• ut serva eum tollere non possit then gives the result

Latin often uses word order for emphasis rather than basic grammar.

Because that would mean something slightly different.

• serva eum non tollit = the slave-girl is not lifting it / does not lift it
• serva eum tollere non possit = the slave-girl cannot lift it

The sentence is not just saying that she does not lift it. It is saying that the sack is so heavy that she is unable to lift it. So possit is important for the idea of ability.

Possit is present subjunctive.

It is present because the sentence is describing a present situation:

• the sack is heavy
• as a result, the slave-girl cannot lift it

If the main sentence were in a past tense, Latin might use a different subjunctive form in the result clause depending on the sequence of tenses. But here the present subjunctive fits the present-time context.