Zkratka je nebezpečná.

Because it agrees with zkratka, which is a feminine singular noun.

In Czech, adjectives must match the noun they describe in:

• gender
• number
• case

Here:

• zkratka = feminine, singular, nominative
• so the adjective must also be feminine, singular, nominative: nebezpečná

The masculine form would be nebezpečný, and the neuter form would be nebezpečné.

Yes, zkratka is feminine.

A very common clue is the ending -a. Many Czech nouns ending in -a are feminine, especially ones like:

• otázka = question
• kniha = book
• zkratka = abbreviation / shortcut

There are exceptions in Czech, but in this case zkratka is a regular feminine noun.

Zkratka can mean more than one thing, depending on context:

• abbreviation
• shortcut

So Zkratka je nebezpečná could mean either:

• The abbreviation is dangerous — less likely in everyday use unless the context is very specific
• The shortcut is dangerous — often the more natural interpretation in ordinary situations

Czech often relies on context to tell you which meaning is intended.

Je is the 3rd person singular present tense of být (to be).

So:

• já jsem = I am
• ty jsi = you are
• on / ona / ono je = he / she / it is

In Zkratka je nebezpečná, je means is.

Unlike in some languages, you normally do need this verb in a standard sentence like this.

In standard Czech, not normally.

So the normal full sentence is:

• Zkratka je nebezpečná.

You may sometimes see or hear shortened, headline-like, poetic, or very informal omissions, but for a learner, the correct standard form is to keep je.

Because Czech has no articles.

English distinguishes:

• a shortcut
• the shortcut

Czech usually just says:

• zkratka

The exact meaning depends on context. So Zkratka je nebezpečná could mean:

• A shortcut is dangerous
• The shortcut is dangerous
• sometimes even Shortcuts are dangerous in a very general sense, though that would more naturally be plural in Czech

Context tells you which one is meant.

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

The sentence structure is:

• Zkratka = subject
• je nebezpečná = predicate

The adjective nebezpečná is also in the nominative because it is a predicate adjective agreeing with the subject.

Because nebezpečnou would be a different case, usually accusative or instrumental feminine singular, depending on context.

After být with an adjective like this, Czech uses the nominative:

• Zkratka je nebezpečná.

So:

• nebezpečná = correct here
• nebezpečnou = not correct in this sentence

Yes. Czech word order is more flexible than English word order.

The neutral order is:

• Zkratka je nebezpečná.

But you might also hear:

• Nebezpečná je zkratka.

That version changes the emphasis, roughly like:

• It is the shortcut that is dangerous
• or it puts extra focus on dangerous

For learners, the original sentence is the best neutral pattern to use.

A simple pronunciation guide:

• Zkratka ≈ ZKRAHT-kah
• je ≈ yeh
• nebezpečná ≈ neh-bez-PECH-naa

A few useful points:

• Czech stress is usually on the first syllable of each word.
• č sounds like ch in chocolate.
• á is a long vowel, so it is held a little longer than short a.

So the stress pattern is roughly:

• ZKRAT-ka je ne-bez-PEČ-ná

The accent mark in á shows that the vowel is long.

So:

• a = short
• á = long

Length matters in Czech pronunciation and can sometimes change meanings or forms.

Also, the letter č is a separate Czech letter, pronounced like ch in chocolate.

You make it negative by changing je to není:

• Zkratka není nebezpečná. = The shortcut is not dangerous.

That is the normal way to negate to be in the present tense.

You would use the plural:

• Zkratky jsou nebezpečné.

Changes:

• zkratka → zkratky = plural noun
• je → jsou = are
• nebezpečná → nebezpečné = adjective agreeing with a feminine plural noun

So Czech changes several words to show the plural.

By form, it is singular, so the most direct reading is:

• The shortcut is dangerous
• or A shortcut is dangerous

If you want a clearly general plural statement, Czech would usually say:

• Zkratky jsou nebezpečné.

So the original sentence is grammatically singular, even if context might sometimes make it feel more general.

It is a regular descriptive adjective meaning dangerous.

Its basic dictionary form is usually the masculine singular:

• nebezpečný

Then it changes depending on the noun:

• nebezpečný = masculine
• nebezpečná = feminine
• nebezpečné = neuter / some plural forms

In your sentence, it takes the feminine form because zkratka is feminine.