In iudicio testes veritatem dicere debent.

Because in can take different cases depending on the idea being expressed.

• in + ablative usually means in, on, or at in the sense of location
• in + accusative usually means into in the sense of motion toward

Here, in iudicio means in court or in a trial, so it expresses location, not motion. That is why iudicio is ablative.

The dictionary form is iudicium, and iudicio is its ablative singular form.

Testes is nominative plural, and it is the subject of the sentence.

The verb debent is third person plural, meaning they ought / they must / they should, so we expect a plural subject. Testes matches that perfectly: witnesses.

Its singular is testis and its plural nominative is testes.

So:

• testis = witness
• testes = witnesses

Because it is the direct object of dicere.

The verb dicere means to say or to speak, and here the thing being said is the truth. In Latin, the direct object normally goes in the accusative case.

So:

• veritas = truth
• veritatem = truth, as the direct object

In other words, the witnesses are supposed to say the truth.

Because debent is being used with an infinitive.

• debent = they ought / they must / they should
• dicere = to say

Latin commonly uses this pattern:

• a finite verb like debet / debent
• followed by an infinitive like dicere

So dicere debent literally means they ought to say or they must say.

This is very similar to English:

• They ought to tell
• They must speak
• They should say

Dicere is the present active infinitive of dico, dicere, meaning to say or to speak.

It is not a finite verb, so it does not by itself tell you who is doing the action. The subject comes from elsewhere in the sentence—in this case, testes—and the finite verb debent gives the person and number.

So the structure is:

• testes = the witnesses
• dicere = to speak / to say
• debent = ought / must

Together: the witnesses ought to say...

Debent is:

• present tense
• active voice
• indicative mood
• third person plural

It comes from debeo, debere, meaning to owe, and by extension to be obliged to, ought to, or must.

So debent means:

• they owe
• or more naturally here, they ought / they must

Because the sentence is about duty or obligation, debent is a very natural choice.

Latin word order is much more flexible than English word order because Latin uses case endings to show each word’s role in the sentence.

English depends heavily on position:

• The witnesses tell the truth

Latin can rely on endings:

• testes is clearly the subject
• veritatem is clearly the object
• iudicio is clearly part of a prepositional phrase

So In iudicio testes veritatem dicere debent is perfectly normal Latin even though it does not follow a strict English-style subject-verb-object pattern.

The placement also gives a natural flow:

• In iudicio sets the scene first
• testes introduces the people involved
• veritatem dicere debent gives the duty

Yes. That is often the most natural English translation.

Latin dicere literally means to say, but English often prefers tell the truth instead of say the truth. So although the Latin wording is built on say, the idiomatic English translation is usually:

• Witnesses must tell the truth in court

This is a good reminder that translation is not always word-for-word. Latin grammar may be straightforward, but natural English sometimes chooses a different expression.

It can suggest either, depending on context.

The noun iudicium can refer to:

• a court proceeding
• a trial
• a judgment
• a law court in a broader sense

So in iudicio may be understood as:

• in court
• at trial
• during the trial

All are reasonable depending on the situation. A learner should mainly notice the grammar: in + ablative gives the setting.

No. Classical Latin has no definite article like English the and no indefinite article like a/an.

So:

• testes can mean witnesses or the witnesses
• veritatem can mean truth or the truth

You understand which is best from context and idiomatic translation. In this sentence, English normally uses the truth, even though Latin does not have a separate word for the.

They are:

• in = in
• iudicium, iudicii = court, trial, judgment
• testis, testis = witness
• veritas, veritatis = truth
• dico, dicere, dixi, dictum = say, speak
• debeo, debere, debui, debitum = owe; ought; must

Knowing the dictionary forms helps you recognize how the actual sentence forms are built:

• iudicio from iudicium
• testes from testis
• veritatem from veritas
• dicere from dico
• debent from debeo