Hastigheten är hög.

Because the basic noun is en hastighet and -en is the definite singular ending.

So:

• en hastighet = a speed
• hastigheten = the speed

In Swedish, the definite article is often attached to the end of the noun instead of being a separate word like English the.

Swedish usually builds definiteness into the noun itself.

So instead of saying something like the speed, Swedish says hastigheten, where the ending -en already means the.

This is very common in Swedish:

• bilen = the car
• dagen = the day
• hastigheten = the speed

Är is the present tense of vara, which means to be.

So:

• är = is / am / are

Unlike English, Swedish does not change this form depending on the subject:

• jag är = I am
• du är = you are
• hastigheten är = the speed is
• de är = they are

So är stays the same.

Because the adjective agrees with the noun.

Here, hastighet is:

• singular
• definite in meaning
• a common-gender noun (en word)

With a common-gender singular noun, the adjective in this kind of sentence is hög.

Compare:

• Hastigheten är hög. = The speed is high.
• Tempot är högt. = The pace is high.
  (tempo is a neuter noun, an ett word)
• Hastigheterna är höga. = The speeds are high.
  (plural)

So the form hög matches the noun here.

Because hög describes the level or value of the speed, while snabb describes something or someone as fast.

So:

• Hastigheten är hög. = The speed is high.
• Bilen är snabb. = The car is fast.

This is similar to English:

• we say high speed
• but a fast car

So Swedish is following the same basic distinction here.

Hastighet is a common-gender noun, also called an en-word.

So you say:

• en hastighet
• hastigheten

A useful pattern: nouns ending in -het are almost always en words in Swedish.

Examples:

• en möjlighet = a possibility
• en frihet = a freedom
• en hastighet = a speed

Yes. This is the standard Swedish order for a simple statement:

• subject: Hastigheten
• verb: är
• adjective/complement: hög

So Hastigheten är hög is the normal order.

Also remember that Swedish main clauses follow the verb-second rule. That means if you put something else first, the verb still stays in second position:

• Nu är hastigheten hög. = Now the speed is high.

Not:

• Nu hastigheten är hög. ✘

A rough guide:

• hastigheten: stress the first syllable, roughly HAS-ti-hee-ten
• är: roughly like air, but shorter
• hög: the ö sound has no exact English equivalent; it is a rounded vowel made with the lips rounded

A few pronunciation notes:

• The ä in är is similar to the vowel in English bed, but not exactly the same.
• The ö in hög is one of the classic Swedish vowel sounds English speakers need to practice.
• hög has a long vowel, so the ö is held a little longer.

Yes, it is perfectly natural, but it sounds a bit factual or technical.

You might hear it in contexts like:

• driving
• traffic signs or warnings
• engineering
• physics
• formal descriptions

In very everyday speech, a Swede might also say something more direct depending on the situation, for example:

• Du kör för fort. = You are driving too fast.
• Farten är hög. = The speed is high.

But Hastigheten är hög is still completely normal Swedish.

Normally, no. That sounds incomplete or unnatural in standard Swedish.

You usually need either:

• an indefinite form: En hastighet ...
• or a definite form: Hastigheten ...

In this sentence, hastigheten is the natural choice because it refers to a specific speed being discussed.

So:

• Hastigheten är hög. ✔
• Hastighet är hög. ✘

Swedish generally does not use bare singular count nouns the way English sometimes can in special contexts.