Si lex nimis gravis esset, multi cives iura sua intellegere non possent.

Because this sentence is a present contrary-to-fact conditional.

• Si lex nimis gravis esset = If the law were too harsh
• multi cives ... non possent = many citizens would not be able...

In Latin, a present unreal condition typically uses the imperfect subjunctive in both clauses:

• si
  • imperfect subjunctive
• main clause + imperfect subjunctive

So:

• esset = imperfect subjunctive of esse
• possent = imperfect subjunctive of posse

This corresponds to English were / would be able in a hypothetical situation.

It is an unreal present condition (also called a contrary-to-fact present condition).

The idea is not simply if the law is too harsh, but rather if the law were too harsh—a hypothetical situation, not a straightforward real one.

Compare:

• Si lex nimis gravis est, ... = If the law is too harsh, ...
  (more neutral / potentially real)
• Si lex nimis gravis esset, ... = If the law were too harsh, ...
  (hypothetical / unreal)

So the Latin is signaling a more imagined or non-actual scenario.

Because it depends on possent.

The verb posse means to be able, and in Latin it is commonly followed by a complementary infinitive—just like English:

• can understand
• are able to understand
• would be able to understand

So:

• non possent intellegere = would not be able to understand

Here, intellegere tells you what they would not be able to do.

There are two different subjects:

1. In Si lex nimis gravis esset, the subject is lex

   • lex = the law

2. In multi cives iura sua intellegere non possent, the subject is multi cives

   • multi cives = many citizens

So the sentence structure is:

• the law would be too harsh
• many citizens would not be able to understand their rights

Lex is nominative singular because it is the subject of esset.

• dictionary form: lex, legis = law
• nominative singular: lex

Since lex is what would be too harsh, it must be in the nominative.

Gravis is an adjective agreeing with lex.

Since lex is:

• feminine
• singular
• nominative

the adjective must also be:

• feminine
• singular
• nominative

For this adjective, the nominative singular masculine and feminine form is the same: gravis.

So:

• lex gravis = a harsh/heavy/serious law
• lex nimis gravis esset = the law would be too harsh

Literally, gravis often means heavy, but in many contexts it is more figurative:

• serious
• severe
• harsh
• burdensome

With lex, it makes natural sense as something like:

• too harsh
• too severe
• too burdensome

So even though the core meaning is related to weight or heaviness, here it describes the law’s severity.

Nimis means too much or simply too.

Here it modifies gravis:

• nimis gravis = too harsh / too severe

So the phrase does not mean very harsh, but specifically excessively harsh.

Iura is the plural of ius.

• ius = right, law, legal right
• iura = rights

In this sentence, it means rights, as in the rights belonging to the citizens.

It is accusative plural because it is the direct object of intellegere:

• iura sua intellegere = to understand their rights

Because sua is the reflexive possessive adjective, and it refers back to the subject of its own clause: multi cives.

So:

• multi cives iura sua intellegere = many citizens to understand their own rights

Latin commonly uses suus, -a, -um when the possessor is the subject of the clause.

If you used eorum, that would usually mean the rights of them in a non-reflexive sense, pointing more to someone else rather than back to the subject itself.

So sua is exactly what you would expect here.

Because it agrees with iura, not directly with cives.

• iura is neuter plural accusative
• therefore the possessive adjective must also be neuter plural accusative
• that form is sua

So:

• iura sua = their rights

Even though the owners are citizens, the adjective’s grammatical agreement is with the noun being described, iura.

Because non is negating the verbal idea would not be able.

• non possent intellegere = would not be able to understand

Latin word order is flexible, but placing non before the finite verb is very common.

You should understand the whole phrase together:

• possent intellegere = could understand
• non possent intellegere = could not understand

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

For example, the following would still mean essentially the same thing:

• Si lex esset nimis gravis, multi cives non possent iura sua intellegere.
• Multi cives, si lex nimis gravis esset, iura sua intellegere non possent.

The original order is perfectly natural, but Latin often moves words for emphasis, rhythm, or style.

Multi is an adjective meaning many, and it agrees with cives.

• multi cives = many citizens

This is the normal and expected placement: adjective before noun. Latin can vary the order, but this arrangement is straightforward and common.

Both words are:

• masculine
• plural
• nominative

because together they form the subject of possent.

Yes. Both spellings are found.

• intellegere
• intelligere

They are just variant spellings of the same verb, meaning to understand.

Many textbooks and dictionaries prefer one spelling over the other, but you should recognize both.

A real or more open condition would usually use the indicative, not the imperfect subjunctive.

For example:

• Si lex nimis gravis est, multi cives iura sua intellegere non possunt.
• If the law is too harsh, many citizens cannot understand their rights.

That version presents the condition as a real possibility or actual situation.

But the original:

• Si lex nimis gravis esset, multi cives iura sua intellegere non possent.

presents it as hypothetical:

• If the law were too harsh, many citizens would not be able to understand their rights.

So the mood change is very important for the sense.