Na dveřích je dlouhý škrábanec.

Because na here means on in the sense of location, and with location it takes the locative case.

So:

• na dveřích = on the door
• na dveře = onto the door / movement toward the door

In this sentence, the scratch is already located there, so Czech uses na + locative.

Dveřích is the locative plural form of dveře.

A very important thing: dveře is a plural-only noun in Czech. Even when English says door in the singular, Czech normally uses dveře.

So the pattern is:

• dictionary form: dveře
• locative after na: na dveřích

This is why na dveři would be wrong here.

Because the verb agrees with škrábanec, not with dveřích.

The structure is basically:

• Na dveřích = location
• je = is
• dlouhý škrábanec = the thing that exists there

So the real subject is škrábanec, which is singular, so Czech uses je.

A useful comparison:

• Dveře jsou otevřené. = The door is open.
  Here dveře is the subject, so the verb is plural.
• Na dveřích je dlouhý škrábanec.
  Here škrábanec is the subject, so the verb is singular.

Because dlouhý has to agree with škrábanec.

Škrábanec is:

• masculine
• inanimate
• singular
• nominative

So the adjective also appears in the matching form:

• dlouhý škrábanec

If the noun changed, the adjective would change too. For example:

• dlouhé dveře = long doors
• dlouhého... = of a long...

This is normal Czech adjective agreement.

Yes. Dlouhý means long, so it describes the scratch's length.

It does not mean:

• deep = hluboký
• big = velký

So:

• dlouhý škrábanec = a long scratch
• hluboký škrábanec = a deep scratch
• velký škrábanec = a big scratch

Because Czech has no articles.

So Czech does not normally have separate words corresponding to English a/an and the. Whether something is definite or indefinite is understood from context.

That means Na dveřích je dlouhý škrábanec could correspond to English ideas like:

• There is a long scratch on the door.
• There’s the long scratch on the door.

In practice, context tells you which one is meant.

Yes. Czech word order is fairly flexible.

This version:

• Na dveřích je dlouhý škrábanec.

is very natural if you are introducing what is on the door. It starts with the location and then presents the new information.

You could also say:

• Dlouhý škrábanec je na dveřích.

That sounds more like you are talking about the scratch and saying where it is.

So the word order changes the focus more than the basic meaning.

The difference is mainly about surface versus inside/in.

• na dveřích = on the door, on its surface
• ve dveřích = in the door or in the doorway

So a scratch goes na dveřích, because it is on the surface.

But something like a lock or window panel could be described with ve dveřích, depending on the situation.

Škrábanec means a scratch or scratch mark.

It is a noun referring to the visible mark. It is related to the verb škrábat, meaning to scratch.

So:

• škrábat = to scratch
• škrábanec = a scratch / scratch mark

In this sentence, it means the mark you can see on the door.

A rough pronunciation guide is:

Na dveřích je dlouhý škrábanec
≈ nah dveh-rzheekh yeh dloh-hee shkraa-ba-nets

A few useful notes:

• ř is the famous Czech sound that does not exist in English; learners often approximate it as something between r and zh
• š sounds like English sh
• ch sounds like the ch in German Bach or Scottish loch
• í and á are long vowels
• Czech stress is usually on the first syllable of each word

So:

• DVE-řích
• DLOU-hý
• ŠKRÁ-ba-nec