Martorul spune adevărul.

Because -ul is the Romanian definite article for many singular masculine and neuter nouns.
So:

• martor = witness
• martorul = the witness

and

• adevăr = truth
• adevărul = the truth

Unlike English, Romanian usually attaches the to the end of the noun instead of putting it before the noun.

Romanian normally uses an enclitic definite article, which means the article is attached to the noun.

So English:

• the witness
• the truth

becomes Romanian:

• martorul
• adevărul

This is one of the big structural differences between English and Romanian.

Spune is the 3rd person singular present form of a spune = to say / to tell.

In this sentence, it matches martorul because martorul is singular:

• eu spun = I say
• tu spui = you say
• el/ea spune = he/she says
• noi spunem = we say
• voi spuneți = you all say
• ei/ele spun = they say

So martorul spune means the witness says / tells.

Romanian present tense can often cover both ideas, depending on context.

So Martorul spune adevărul can mean:

• The witness tells the truth
• The witness is telling the truth

If there is no extra context, English usually chooses whichever sounds more natural in the situation.

Because adevărul is a direct object, but it is not the kind of direct object that normally takes pe.

In Romanian, pe is commonly used before specific animate direct objects, especially people:

• Văd băiatul can occur, but
• Îl văd pe băiat is the more typical marked form with a person

But adevărul = the truth is inanimate, so no pe is needed:

• Martorul spune adevărul

That is completely normal.

It is the direct object, so functionally it is in the accusative.

However, in this sentence, the form adevărul looks the same as the nominative/accusative definite singular form. That is very common in Romanian.

A useful point for learners: Romanian still has cases, but you often do not see a dramatic change in the noun form in simple sentences like this one.

• martor is masculine
• adevăr is neuter

Romanian neuter nouns behave like:

• masculine in the singular
• feminine in the plural

So in the singular, adevărul takes the same kind of definite article ending you often see with masculine nouns.

This is the most neutral, standard word order: subject + verb + object.

• Martorul = subject
• spune = verb
• adevărul = object

Romanian word order is more flexible than English, so other orders are possible for emphasis or style. But for a basic statement, Martorul spune adevărul is the natural default.

A rough English-friendly guide is:

• martorul ≈ MAR-to-rul
• spune ≈ SPOO-neh
• adevărul ≈ a-de-VUH-rul

A few helpful pronunciation notes:

• ă is a short neutral vowel, like the a in sofa
• u is like oo in boot, but shorter
• Romanian r is usually trilled or tapped more than in English

The main stress is roughly:

• MARtorul
• SPUne
• adeVĂrul

It would be:

Un martor spune un adevăr.

But that sounds less natural in many contexts, because truth is very often used as a definite idea in Romanian here: adevărul = the truth.

More naturally:

• martor = a witness
• un martor = a witness
• martorul = the witness

and

• adevăr = truth
• un adevăr = a truth
• adevărul = the truth

No. Romanian also has verbs like a zice, which can sometimes mean to say. But a spune adevărul is a very common and natural expression for to tell the truth.

So for this sentence, spune adevărul is exactly the kind of phrase learners should remember as a useful chunk.

You put nu before the verb:

Martorul nu spune adevărul.

That means The witness does not tell the truth or The witness isn’t telling the truth.

This is the normal Romanian way to negate a verb:

• spune = says / tells
• nu spune = does not say / does not tell