Diagnosta porównał dowód rejestracyjny z tablicą rejestracyjną.

Polish does not have articles like the or a/an.
So Diagnosta porównał dowód rejestracyjny z tablicą rejestracyjną can mean:

• The inspector compared the registration document with the registration plate
• An inspector compared a registration document with a registration plate

Which one is intended depends on context.

Because some Polish masculine nouns end in -a. This is normal and fairly common, especially for words referring to people, for example:

• mężczyzna = man
• kolega = male friend/colleague
• diagnosta = diagnostician / inspector

Even though diagnosta ends in -a, it is grammatically masculine when it refers to a man. You can see that from the verb:

• porównał = masculine singular past
• not porównała = feminine singular past

Porównał is:

• past tense
• 3rd person singular
• masculine
• perfective

Its infinitive is porównać.

The perfective aspect shows a completed action: he compared them, finished doing it.

A related imperfective verb is porównywać. For example:

• porównywał = he was comparing / used to compare

So in your sentence, porównał presents the comparison as a completed event.

In Polish, adjectives often come before nouns, but in many fixed official, technical, or administrative expressions, the adjective comes after the noun.

So these are standard set expressions:

• dowód rejestracyjny
• tablica rejestracyjna

This is similar to how some bureaucratic or technical labels become conventional. Putting the adjective first would sound less standard here.

Because dowód rejestracyjny is the direct object of the verb porównał, so it is in the accusative case.

However, for masculine inanimate singular nouns, the accusative is usually the same as the nominative. That is why the form does not visibly change:

• nominative: dowód rejestracyjny
• accusative: dowód rejestracyjny

So even though it is the object, it looks like the dictionary form.

Because the verb pattern is:

• porównać coś z czymś = to compare something with something

After z in this meaning, Polish uses the instrumental case.

So:

• tablica rejestracyjna → nominative
• z tablicą rejestracyjną → instrumental

The endings change like this:

• tablica → tablicą
• rejestracyjna → rejestracyjną

No. z is a very common preposition with more than one meaning.

Two important patterns are:

• z + instrumental = with
• z + genitive = from / off / out of

Examples:

• z tablicą = with the plate
• z samochodu = from the car

In your sentence, it is z + instrumental because the meaning is compare X with Y.

Because adjectives in Polish must agree with the noun in:

• gender
• number
• case

Here you have two different nouns in two different grammatical roles:

• dowód rejestracyjny

  • dowód is masculine inanimate singular
  • here it is accusative, but it looks like nominative
  • so the adjective is rejestracyjny

• z tablicą rejestracyjną

  • tablica is feminine singular
  • after z it becomes instrumental
  • so the adjective is rejestracyjną

Yes, it is the standard pattern.

• porównać coś z czymś = compare something with something

There is also porównać coś do czegoś, but that often means more compare/liken something to something, especially in a more figurative or rhetorical sense.

So for a literal comparison of two documents or objects, porównać coś z czymś is the natural choice.

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

The sentence you have is a neutral, natural order:

• Diagnosta porównał dowód rejestracyjny z tablicą rejestracyjną.

But other orders are possible for emphasis, for example:

• Dowód rejestracyjny diagnosta porównał z tablicą rejestracyjną.
• Z tablicą rejestracyjną diagnosta porównał dowód rejestracyjny.

These alternatives are grammatically possible, but the original version is the most straightforward and neutral.

It is in the nominative singular because it is the subject of the sentence, the person doing the action.

So the structure is:

• Diagnosta = subject, nominative
• porównał = verb
• dowód rejestracyjny = direct object, accusative
• z tablicą rejestracyjną = prepositional phrase with instrumental

That is a very useful pattern to recognize in Polish sentences.