Learn Hardcore Polish: Tablica rejestracyjna jest czysta.

Polish has no articles, so there is no separate word for a or the. Whether tablica rejestracyjna means a license plate or the license plate depends on context. In a normal real-life situation, this sentence will often be understood as The license plate is clean.

What are the grammatical roles of the words in this sentence?

Tablica rejestracyjna is the subject, jest is the verb, and czysta is the adjective describing the subject.

So the structure is:

[subject] + [form of być / to be] + [adjective]

In other words, it works very much like The license plate is clean in English.

Why is rejestracyjna after tablica instead of before it?

Because tablica rejestracyjna is the normal fixed expression for license plate / registration plate in Polish. Although adjectives often come before nouns in Polish, some established official or technical expressions place the adjective after the noun. This is one of those cases.

So learners should treat tablica rejestracyjna as a set phrase rather than trying to apply a simple word-order rule to every adjective.

Why do rejestracyjna and czysta both end in -a?