Zloděj je neznámý.

Czech has no articles, so it does not use separate words like the or a/an.

So zloděj can mean:

• the thief
• a thief

Which one is meant depends on the context. In a standalone sentence like this, English might translate it as The thief is unknown or A thief is unknown, but context usually makes one more natural.

• zloděj = a noun, the subject
• je = is, the 3rd person singular of být (to be)
• neznámý = an adjective meaning unknown, describing the subject

So the structure is very similar to English:

subject + is + adjective

In standard Czech, je is normally required in this kind of sentence.

So:

• Zloděj je neznámý. = correct
• Zloděj neznámý. = not a normal complete sentence in standard Czech

This is different from some other Slavic languages where the present-tense to be can be omitted.

Zloděj is a masculine noun. Because the adjective describes it, the adjective must agree with it in:

• gender
• number
• case

Here:

• zloděj is masculine singular
• so the adjective is neznámý, the masculine singular form

If the noun changed, the adjective would change too.

For example:

• žena je neznámá = the woman is unknown
• město je neznámé = the town is unknown

It is in the nominative case, because it is the subject of the sentence.

Also, after být in this kind of sentence, the adjective neznámý is in the nominative too.

So this is a straightforward nominative subject + nominative predicate adjective pattern.

In this sentence, neznámý is clearly an adjective meaning unknown.

But in Czech, adjectives can sometimes be used substantively, meaning they function like nouns. For example, neznámý can also mean an unknown man / stranger, depending on context.

Here, though, because it follows je and describes zloděj, it is best understood as an adjective.

Yes. Czech word order is more flexible than English because case endings and agreement carry a lot of grammatical information.

The neutral order is:

• Zloděj je neznámý.

But you may also see:

• Neznámý je zloděj.
• Neznámý zloděj je... in a larger sentence

Changing the order usually changes emphasis, not the basic meaning. The original order is the most normal and neutral if you are simply stating a fact.

A rough guide is:

• zlo sounds like zlo
• děj is approximately like dyay or dyehy, but not exactly English

A more careful breakdown:

• ě after d makes the d sound softer, close to ď
• so dě is pronounced roughly like ďe
• j is pronounced like English y in yes

So zloděj is approximately ZLO-dyey or ZLO-ďej, with the stress on the first syllable:

• ZLO-děj

As always, the English approximation is imperfect, but that is the basic idea.

The ý shows a long í-like vowel. In pronunciation, y and i are pronounced the same in modern standard Czech, but the spelling still matters for grammar.

So in neznámý:

• the final vowel is long
• the -ý ending also helps mark this as a masculine singular adjective form

So the letter is important both for pronunciation length and for grammar/spelling.

Yes. If the subject were feminine, the adjective would change.

For example:

• Zlodějka je neznámá. = the female thief is unknown

Here:

• zlodějka is feminine
• neznámá is the feminine singular form

This is a good example of adjective agreement in Czech.

You would change both the noun and the adjective:

• Zloději jsou neznámí. = the thieves are unknown

Changes:

• zloděj → zloději = plural
• je → jsou = are
• neznámý → neznámí = masculine animate plural adjective

This is useful because zloděj refers to people, so in the plural it uses the masculine animate pattern.

Yes. Zloděj is masculine animate because it refers to a person.

That matters especially in:

• plural adjective forms
• plural case endings
• some numeral constructions

In this singular sentence, the animate/inanimate distinction is not very visible, but it becomes much more important in other forms, such as:

• zloději
• neznámí