Testis iurat se verum dicere.

Because testis is the subject of iurat.

• testis = the witness
• iurat = swears

In Latin, the subject of a finite verb is normally in the nominative case, just as in English the subject is the one doing the action.

Testis is a third-declension noun. Its basic dictionary form is:

• testis, testis (m./f.) = witness

A learner might expect a nominative ending like -us or -a, but many third-declension nouns have other endings, and -is is very common there.

Iurat is:

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

It means he/she swears or the witness swears.

Why singular? Because the subject testis is singular.

Se is the reflexive pronoun, meaning himself/herself/themselves depending on context.

Here it refers back to the subject testis. So:

• testis iurat se... = the witness swears that he...

Latin uses se when the person inside the subordinate idea is the same as the subject of the main verb.

So se does not mean some other person here. It means the witness himself.

Because Latin is using an indirect statement construction, often called accusative + infinitive.

After verbs like:

• dicit = says
• putat = thinks
• scit = knows
• iurat = swears

Latin often expresses that ... with:

• a subject in the accusative
• and a verb in the infinitive

So in se verum dicere:

• se = accusative subject of the infinitive
• dicere = infinitive

This is the Latin way of saying that he tells / is telling the truth.

Because after iurat, Latin commonly uses indirect statement rather than a clause with that.

English says:

• The witness swears that he is telling the truth.

Latin often says this instead as:

• The witness swears himself to tell the truth

That literal English sounds strange, but grammatically it helps show the Latin structure:

• iurat = main verb
• se ... dicere = indirect statement

So dicere is infinitive because it belongs to that indirect statement construction.

Here verum means the truth or what is true.

Grammatically, it is the neuter singular accusative of the adjective verus, vera, verum = true. Latin often uses a neuter adjective by itself as a noun-like word:

• verum = the true thing, the truth

So verum dicere is an idiomatic way to say:

• to speak the truth
• to tell the truth

Latin can express truth in more than one way.

• veritas = truth, truthfulness
• verum = the truth, what is true

In this sentence, verum dicere is a very natural idiom meaning to tell the truth.

So verum is not wrong or unusual here; it is simply a common Latin way to express the idea.

No. The sentence does not mean the witness swears that he is true. If Latin wanted to say that, it would more naturally use something like:

• testis iurat se verum esse

But here we have:

• se verum dicere

That means that he says/speaks the truth, not that he is true.

So verum is the object of dicere, not a predicate adjective with an omitted esse.

The sentence breaks down like this:

• testis = subject of the main verb
• iurat = main verb, swears
• se = accusative subject of the infinitive
• verum = object of dicere
• dicere = infinitive in indirect statement

So the structure is:

• [The witness] [swears] [that he tells the truth].

This is a very standard Latin pattern.

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

For example, these would still be understandable Latin:

• Se verum dicere testis iurat.
• Testis se verum dicere iurat.

The version Testis iurat se verum dicere is perfectly natural, but Latin does not depend on word order as heavily as English does.

Still, word order can affect emphasis. Putting a word earlier or later can make it stand out more.

Because in this kind of construction Latin usually does not use a separate word for that.

English says:

• The witness swears that he tells the truth.

Latin instead uses the accusative + infinitive construction:

• testis iurat se verum dicere

So the idea of English that is built into the grammar, not expressed by a separate word.

A very literal translation would be something like:

• The witness swears himself to speak the truth.

That is not good normal English, but it helps reveal the Latin grammar:

• se = himself in the accusative
• dicere = to speak / to say

Then you convert it into natural English:

• The witness swears that he is telling the truth
• or The witness swears that he tells the truth