Diagnosta dopuścił pojazd do ruchu.

In Polish, some masculine personal nouns end in -a. Diagnosta is one of them.

So even though the noun looks a bit like a feminine noun, it takes masculine agreement:

• ten diagnosta = this male inspector
• Diagnosta dopuścił... = the verb is masculine past tense

This is similar to other masculine nouns like kolega or mężczyzna.

Here, diagnosta means a vehicle inspector / vehicle examiner / diagnostic technician, not a medical diagnostician.

In the context of pojazd and do ruchu, it is someone who checks a vehicle and officially decides whether it can be used on the road.

There isn’t one, because Polish has no articles.

So:

• diagnosta can mean a diagnostician or the diagnostician
• pojazd can mean a vehicle or the vehicle

The exact meaning depends on context.

Dopuścił is:

• past tense
• 3rd person singular
• masculine
• from the verb dopuścić

It means something like approved, allowed, or admitted.

The -ł ending is the usual marker of masculine singular past tense in Polish.

This is a question of aspect.

• dopuścić = perfective
  It describes a completed action.
• dopuszczać = imperfective
  It describes an ongoing, repeated, or non-completed action.

So:

• Diagnosta dopuścił pojazd do ruchu = the inspector approved the vehicle; the decision is complete.
• Diagnosta dopuszczał pojazd do ruchu would suggest something like was approving, used to approve, or a less clearly completed action.

In this sentence, the perfective verb is natural because it refers to a finished official decision.

Because pojazd is the direct object of the verb dopuścił, so it is in the accusative case.

But pojazd is a masculine inanimate noun, and for that group:

• nominative singular = pojazd
• accusative singular = pojazd

So the form looks unchanged.

If it were masculine animate, the accusative would usually look different.

Here do ruchu is part of a very common official expression:

dopuścić pojazd do ruchu

This means:

• to approve a vehicle for road use
• to allow a vehicle into traffic
• to certify that a vehicle may legally be used

So ruch here is not just general movement. In this context, it means traffic or circulation on the road.

Because the preposition do normally requires the genitive case.

So:

• basic form: ruch
• after do: do ruchu

That is why the sentence has do ruchu, not do ruch.

Yes, if you specifically mean a car.

• pojazd = vehicle (broader term)
• samochód = car

So:

• Diagnosta dopuścił pojazd do ruchu = neutral, official, broad
• Diagnosta dopuścił samochód do ruchu = specifically a car

In formal or legal language, pojazd is often preferred.

Yes. Polish word order is fairly flexible because the case endings show the grammatical roles.

The neutral order here is:

• Diagnosta dopuścił pojazd do ruchu.

But you could also say:

• Diagnosta dopuścił do ruchu pojazd.

That still means the same thing, but the emphasis changes slightly. The original version is the most straightforward and neutral.

The past tense verb would change to a feminine form:

• Diagnostka dopuściła pojazd do ruchu.

The key change is:

• dopuścił = masculine
• dopuściła = feminine

So the verb ending tells you the gender of the subject in the past tense.