Learn Dutch: Ik heb het ongeluk op de weg gezien. - I saw the accident on the road.

In Dutch, when you use a two-part verb tense like the present perfect, the helper verb (heb) takes the normal second position in the sentence. The past participle (gezien) is pushed all the way to the end. Everything else gets sandwiched in between.

Could I say "Ik heb gezien het ongeluk op de weg" like in English?

No, that sounds very unnatural in Dutch. Because gezien must go to the end of the clause, the direct object (het ongeluk) and the location phrase (op de weg) have to come before it.

Does ongeluk only mean a physical accident, like a car crash?

It means accident here, but it literally translates to un-luck or bad luck (geluk means luck or happiness). It is used for both physical accidents and general misfortune.

How does having gezien at the end relate to the verb-final word order with omdat?

In this regular main sentence, only the past participle (gezien) goes to the end while heb stays in the second position. When you use a subordinating word like omdat (because), it pushes all the verbs to the end of their clause. Getting used to putting participles at the end here is great practice for those strict verb-final rules!