Si quis mendacium in iudicio dicat, iudex eum damnare potest; si innocens sit, eum absolvere debet.

Because after si, Latin normally uses quis instead of aliquis.

So:

• aliquis = someone, anyone
• but after words like si, nisi, num, and ne, the ali- usually drops off

That means si quis simply means if anyone.

In this sentence, quis is nominative singular and is the subject of dicat.

Dicat and sit are present subjunctive forms:

• dicat = he may say / should say / were to say
• sit = he may be / should be / were to be

In a si clause, the subjunctive often gives the condition a more indefinite or potential sense. So the Latin is not stating a plain fact like if someone tells a lie in the most direct way; it is more like:

• if anyone should tell a lie
• if he should be innocent

In natural English, we often still translate this more simply as if anyone tells a lie and if he is innocent.

It is a more general or potential kind of condition rather than a straightforward present factual condition.

The use of the present subjunctive in the si clauses suggests something like:

• if anyone were to tell a lie...
• if he were to be innocent...

But the main clauses use indicative verbs, potest and debet, because they state what the judge can do or must do. So the sentence sounds like a legal or general principle:

• under that condition, the judge has the power to condemn
• under the other condition, the judge has the duty to acquit

Mendacium is accusative singular.

It is the direct object of dicat, because dicere can take a thing said as its object. So:

• mendacium dicere = to tell a lie or more literally to say a falsehood

This is a very normal Latin expression.

Iudicio is ablative singular, used after in.

With in, Latin usually works like this:

• in
  • ablative = in, on, at a place
• in
  • accusative = into, onto a place

So in iudicio means in court or at trial.

The noun iudicium can mean judgment, trial, or court proceedings, depending on context.

Because Latin often leaves subject pronouns unstated when the verb already shows the person and number.

Here, sit is third person singular, so Latin does not need an explicit he. The sense is understood from the ending and from the context.

So si innocens sit means:

• if he is innocent
• or more literally, in this kind of sentence, if he should be innocent

The understood subject is the same person already under discussion.

Innocens is a third-declension adjective, and its nominative singular masculine and feminine form is simply innocens.

It may look unusual if you are expecting a first/second-declension adjective like bonus, bona, bonum. But innocens belongs to a different pattern.

Here it is nominative because it is a predicate adjective with sit:

• he is innocent
• si innocens sit = if he is innocent

If it were accusative singular, you would see innocentem instead.

Because they depend on potest and debet.

This is the same basic idea as in English:

• can condemn
• must acquit
• ought to acquit

In Latin:

• potest damnare = can condemn
• debet absolvere = must/ought to acquit

So damnare and absolvere are complementary infinitives. Their subject is understood to be the same as the subject of the finite verb, namely iudex.

Eum is accusative singular masculine.

It is the direct object of the infinitives:

• eum damnare = to condemn him
• eum absolvere = to acquit him

It refers to the person introduced earlier by quis.

It is repeated because each clause has its own verb phrase, and Latin often repeats a pronoun for clarity and balance, especially in a neatly parallel sentence like this one. English might also repeat him in the same way.

Both are third person singular present indicative active:

• potest from posse = is able, can
• debet from debere = owes, ought, must

They match iudex, which is singular:

• iudex ... potest = the judge can
• iudex ... debet = the judge must/ought to

So the finite verbs of the main clauses are both present indicative.

Latin word order is much more flexible than English word order because the endings show how words function.

So Latin can say:

• iudex eum damnare potest instead of forcing only one order like English

Here the order is quite natural:

• iudex first gives the subject
• eum comes before the infinitive it belongs with
• damnare and absolvere come near the end
• potest and debet finish the main statements neatly

The semicolon also helps show the parallel structure:

• if anyone should tell a lie ... the judge can condemn him
• if he should be innocent ... he must acquit him

Many Latin texts use classical spelling, where i does the job later done by both i and j in English-based spelling conventions.

So:

• iudex = later often written judex
• iudicium = later often written judicium

The same thing happens with u and v in some editions.

So this is mostly a spelling convention, not a difference in meaning.