Pe când martorul traversa strada, a văzut coliziunea.

Pe când means something like while or at the time when in this sentence. It introduces a background action that was in progress.

So:

• Pe când martorul traversa strada... = While the witness was crossing the street...

Compared with când, pe când often sounds a bit more literary or more explicitly contrastive/background-setting. In everyday speech, many speakers would also say:

• Când martorul traversa strada, a văzut coliziunea.
• În timp ce martorul traversa strada, a văzut coliziunea.

All are possible, but pe când has a slightly more narrative feel.

Because -ul is the Romanian definite article attached to the end of the noun.

• martor = witness / a witness
• martorul = the witness

Romanian usually puts the definite article at the end of the noun, unlike English, which uses a separate word:

• băiat = boy
• băiatul = the boy
• martor = witness
• martorul = the witness

So martorul means the witness.

Because traversa is the imperfect tense, which is used here for an action that was ongoing in the past.

• traversa = was crossing / used to cross
• a traversat = crossed / has crossed

In this sentence, the crossing is the background action in progress, and the seeing is the main event that happened during it:

• martorul traversa strada = the witness was crossing the street
• a văzut coliziunea = he/she saw the collision

This is a very common Romanian pattern:

• imperfect = background, ongoing action
• perfect compound = completed event

So the structure is similar to English:

• While the witness was crossing the street, he/she saw the collision.

A văzut is the perfectul compus form of the verb a vedea (to see).

It is made of:

• the auxiliary a = has
• the past participle văzut = seen

So literally, it is historically similar to has seen, but in modern Romanian it is also the normal way to say simple past events in everyday language:

• a văzut = saw / has seen, depending on context

In this sentence, the natural English translation is saw.

A full mini-paradigm:

• am văzut = I saw
• ai văzut = you saw
• a văzut = he/she saw
• am văzut = we saw
• ați văzut = you all saw
• au văzut = they saw

Here, strada is the definite form of stradă.

• stradă = street / a street
• strada = the street

So:

• traversa strada = was crossing the street

Again, Romanian attaches the definite article to the noun instead of using a separate word like the.

Also note the dictionary form is stradă, but in the definite singular it becomes strada.

Coliziunea means the collision, while o coliziune means a collision.

• coliziune = collision
• o coliziune = a collision
• coliziunea = the collision

The definite form is used because the speaker is presenting it as a specific event, not just any collision. In many contexts, Romanian prefers the definite form when referring to the particular event being described.

That said, depending on context, a văzut o coliziune could also be possible if the meaning were more like he/she saw a collision for the first time in the story, with less emphasis on its being already identifiable.

The comma separates the subordinate clause from the main clause.

• Pe când martorul traversa strada = subordinate time clause
• a văzut coliziunea = main clause

So the comma works much like in English:

• While the witness was crossing the street, he saw the collision.

Romanian punctuation often uses a comma after an introductory subordinate clause like this.

Yes. The subject of a văzut is still martorul.

Romanian often leaves subjects unstated because the verb form already gives person and number, but here the noun martorul appears in the first clause and continues as the understood subject of the second clause.

So the sentence means:

• While the witness was crossing the street, the witness saw the collision.

Romanian does not need to repeat martorul.

Yes, Romanian word order is fairly flexible, though some versions sound more natural than others.

For example, you could also say:

• Martorul a văzut coliziunea pe când traversa strada.
• Pe când traversa strada, martorul a văzut coliziunea.

These are all understandable.

However, the original version is very natural because it first sets the scene, then gives the main event:

• Pe când martorul traversa strada, a văzut coliziunea.

That is a common narrative order.

Yes, a traversa strada is completely normal and clear. It is a fairly standard, neutral way to say to cross the street.

You may also hear:

• a trece strada
• a traversa strada

Both are used. A traversa can sound a bit more formal or precise, while a trece is often more everyday.

So:

• Martorul traversa strada = the witness was crossing the street
• Martorul trecea strada = also possible, with a very similar meaning

A little, yes.

Nothing in it is unnatural, but a few choices make it sound slightly more formal or narrative:

• pe când instead of a simpler când or în timp ce
• coliziunea instead of a more everyday word like accidentul, depending on context
• a traversa can sound a bit more formal than a trece

A more everyday version might be:

• În timp ce martorul traversa strada, a văzut accidentul.

But the original sentence is still perfectly good Romanian.

A rough pronunciation guide:

• Pe când ≈ pe kund
• martorul ≈ MAR-to-rul
• traversa ≈ tra-ver-SA
• strada ≈ STRA-da
• a văzut ≈ uh vuh-ZOOT
• coliziunea ≈ ko-li-zee-U-ne-a

A couple of useful notes:

• ă is a short central vowel, somewhat like the a in about
• â and î are Romanian-specific central vowels; in când, the â does not sound like English u, but kund is a rough approximation
• z is pronounced like English z
• j in Romanian sounds like the s in measure, but there is no j in this sentence

If you want, I can also break the whole sentence down word by word with pronunciation and grammar tags.