Toto prostředí je nebezpečné.

Toto means this.

It is the neuter singular form of the demonstrative pronoun ten (that / the one), used here because prostředí is a neuter singular noun.

So:

• ten = this/that (masculine)
• ta = this/that (feminine)
• to / toto = this/that (neuter)

In this sentence, toto prostředí = this environment / this setting.

In everyday speech, many speakers would more often say tohle prostředí than toto prostředí, but toto is completely correct and a bit more formal or written in tone.

Because it has to agree with prostředí.

In Czech, adjectives change their endings to match the noun’s:

• gender
• number
• case

Here, prostředí is:

• neuter
• singular
• nominative (the subject of the sentence)

So the adjective must also be neuter singular nominative, which gives:

• nebezpečný = masculine
• nebezpečná = feminine
• nebezpečné = neuter

That is why the sentence has nebezpečné.

Prostředí is a neuter noun meaning things like:

• environment
• surroundings
• setting
• sometimes milieu

In this sentence, it means the environment or surroundings being talked about.

Even though it ends in -í, it is singular here: this environment.

Here it is singular.

You can tell from the rest of the sentence:

• toto = neuter singular
• je = is
• nebezpečné = neuter singular adjective form

So the whole sentence clearly means This environment is dangerous, not These environments are dangerous.

Je means is. It is the 3rd person singular form of být (to be).

So:

• je = is

In the present tense, Czech normally does use the verb to be in sentences like this, so you should keep je here.

That means:

• Toto prostředí je nebezpečné. = correct
• Toto prostředí nebezpečné. = not standard Czech

Because Czech has no articles.

English uses:

• the
• a / an

Czech does not. The meaning is understood from context.

So prostředí can mean:

• environment
• the environment
• an environment

Here, toto already makes it specific, so toto prostředí naturally means this environment.

Yes. That is a very natural everyday alternative.

Both mean This environment is dangerous:

• Toto prostředí je nebezpečné.
• Tohle prostředí je nebezpečné.

A rough difference:

• toto = a bit more formal, written, careful
• tohle = more common in everyday spoken Czech

So if you are learning conversational Czech, tohle is very useful.

Yes, Czech word order is fairly flexible.

The neutral version is:

• Toto prostředí je nebezpečné.

But you may also see:

• Tohle prostředí je nebezpečné.
• Prostředí je nebezpečné. = The environment is dangerous / This environment is dangerous
• Nebezpečné je toto prostředí. = more emphasis on dangerous
• Toto je nebezpečné prostředí. = This is a dangerous environment

The basic meaning stays similar, but word order can shift emphasis.

They are close, but not identical in structure.

• Toto prostředí je nebezpečné.
  = This environment is dangerous.
  Here, this environment is the subject, and dangerous is the description.

• Toto je nebezpečné prostředí.
  = This is a dangerous environment.
  Here, this is the subject, and a dangerous environment is what it is identified as.

In English, the difference is small, and in Czech both are common.

Je is pronounced roughly like ye.

So the sentence begins something like:

• TO-to PRO-stře-dí ye NE-bez-peč-né

A learner-friendly approximation would be:

• TOH-toh PROS-tryeh-dee yeh NEH-bez-pech-neh

The exact Czech sounds are a bit different from English, but je = ye is a good starting point.

Prostředí is one of those words learners often notice because of ř.

A rough breakdown:

• pro-
• stře-
• dí

The tricky part is ř, a special Czech sound with no exact English equivalent. It is often described as something between r and zh.

If you cannot say ř yet, do not worry too much at first. A softened r is often an acceptable beginner approximation.

Also remember that Czech stress is usually on the first syllable of each word:

• PROstředí

No. Nebezpečný / nebezpečná / nebezpečné can describe many kinds of danger, including:

• physical danger
• health risk
• political or social danger
• risky situations in general

So Toto prostředí je nebezpečné could refer to:

• a physically unsafe place
• a toxic social atmosphere
• a harmful working environment
• a dangerous natural setting

The exact meaning depends on context.

Not always.

Depending on context, prostředí can mean:

• environment
• surroundings
• setting
• atmosphere
• milieu

For example:

• pracovní prostředí = work environment
• rodinné prostředí = family environment / home environment
• životní prostředí = the environment in the ecological sense

So in this sentence, environment is a good translation, but setting or surroundings might also fit depending on context.

You would simply add není (is not):

• Toto prostředí není nebezpečné.
• This environment is not dangerous.

Here:

• je = is
• není = is not

That is the normal way to negate this kind of sentence in Czech.