Číselná kombinace není správná.

A word-by-word breakdown is:

• číselná = numerical / number-related
• kombinace = combination
• není = is not
• správná = correct / right

So the whole sentence means something like The number combination is not correct or The numeric combination is not correct.

Because they both agree with kombinace, which is a feminine singular noun.

In Czech, adjectives have to match the noun they describe in:

• gender
• number
• case

Here:

• kombinace is feminine
• it is singular
• it is in the nominative

So both adjectives take the feminine singular nominative ending -á:

• číselná kombinace
• kombinace není správná

This agreement happens even with správná, although in English we might think of it as part of the verb phrase is correct. In Czech, it still matches the subject.

Není is the 3rd person singular negative form of the verb být (to be).

Compare:

• je = is
• není = is not

So:

• Číselná kombinace je správná. = The number combination is correct.
• Číselná kombinace není správná. = The number combination is not correct.

Czech often builds negation directly into the verb, instead of adding a separate word like English not.

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

The subject is the thing being talked about:

• Číselná kombinace = the thing that is not correct

Also, správná is in the nominative too, because after být (to be) Czech normally uses a nominative predicate adjective:

• kombinace — nominative feminine singular
• správná — nominative feminine singular

You usually learn the gender of nouns as part of the vocabulary item, but there are also patterns.

Kombinace is feminine, and many Czech nouns ending in:

• -ace
• -ice

are feminine.

So:

• kombinace = feminine
• therefore adjectives matching it must be feminine too:
  • číselná
  • správná

This is something worth memorizing with the noun:

• ta kombinace = the combination

Czech has no articles like English a / an / the.

So:

• kombinace can mean a combination or the combination
• the exact meaning depends on context

In this sentence, English will usually translate it as the number combination is not correct or simply the code is not correct, depending on the situation.

Yes, it is natural, and it means something like:

• number combination
• numerical combination
• sometimes, depending on context, even numeric code

But Czech uses kombinace fairly literally as combination. In real-life contexts, another phrase might also be used depending on what exactly is meant:

• číselný kód = numeric code
• kombinace čísel = combination of numbers

So číselná kombinace není správná is perfectly understandable, but the best English translation may vary by context.

Czech word order is more flexible than English, because endings show grammatical relationships.

The neutral order here is:

• Číselná kombinace není správná.

But other word orders are possible for emphasis, for example:

• Správná číselná kombinace to není. = That is not the correct number combination.
• Správná není číselná kombinace. = unusual, but possible in a special context

Even though Czech allows flexibility, the original sentence is the most natural straightforward version.

A simple pronunciation guide:

• Číselná ≈ CHEE-sel-naa
• kombinace ≈ kom-bi-NA-tse
• není ≈ NE-nee
• správná ≈ SPRAAV-naa

A few helpful points:

• č sounds like ch in church
• í is a long ee
• á is a long aah
• c in Czech sounds like ts
• stress is usually on the first syllable of each word:
  • ČÍ-sel-ná
  • KOM-bi-na-ce
  • NE-ní
  • SPRÁV-ná

Because správná means correct / right in the sense of being accurate or valid.

• správná kombinace = the correct combination
• dobrá kombinace = a good combination

So if you mean that the number sequence entered is wrong, správná is the right adjective.

Compare:

• To není správné. = That isn’t correct.
• To není dobré. = That isn’t good.

Those are similar in English sometimes, but not the same in Czech.

Yes. A common alternative is:

• Číselná kombinace je špatná.

That means The number combination is bad / wrong.

The difference is:

• není správná = is not correct
  More neutral, often a bit more formal or system-like
• je špatná = is wrong / bad
  More direct

In many situations, both are possible. For things like entering a code, není správná sounds very natural.

In this sentence, the subject is needed because it tells you what is not correct.

Czech often omits subject pronouns like he / she / it because the verb ending gives that information, but here the noun phrase itself is important:

• Číselná kombinace není správná.

You could not just say:

• Není správná.

unless the listener already knows what it refers to. On its own, that would mean It isn’t correct, but the subject would be implied from context.