Instruktor ukazuje správné držení těla.

Because it agrees with držení, which is a neuter singular noun.

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

• gender
• number
• case

Here:

• držení = neuter singular
• it is the direct object of the sentence, so it is in the accusative
• for neuter singular, the adjective form is správné

So:

• správné držení = correct posture / correct holding

A learner might expect something like správný or správnou, but those would match different noun types:

• správný = masculine
• správnou = feminine accusative singular

The whole phrase správné držení těla is the direct object of ukazuje, so the main noun držení is in the accusative.

Breakdown:

• Instruktor = nominative, the subject
• ukazuje = verb
• správné držení těla = accusative, what the instructor is showing

However, inside that phrase:

• držení is accusative
• těla is genitive, because it means of the body

So the structure is:

• ukazuje co? → správné držení
• držení čeho? → těla

Because držení těla literally means holding of the body or body posture.

In Czech, one noun often connects to another noun using the genitive where English would often use:

• of
• or a noun used as an adjective

So:

• tělo = body
• těla = of the body

This is very common in Czech:

• barva auta = the color of the car
• část knihy = part of the book
• držení těla = posture, literally holding of the body

Yes. Držení comes from the verb držet and literally means holding or keeping. But in Czech, držení těla is a normal and common expression for posture.

So even though the literal meaning is something like:

• the holding of the body

the natural English meaning is:

• posture
• body posture

This is a good example of a phrase that should be learned as a chunk:

• držení těla = posture

Yes. Ukazuje is the 3rd person singular present tense of ukazovat.

It can mean either:

• shows
• is showing

Czech present tense often covers both the simple present and the English progressive, depending on context.

So this sentence could be translated as:

• The instructor shows correct posture.
• The instructor is showing correct posture.

Both are possible. The context tells you which sounds better in English.

Because ukazuje is the imperfective verb, while ukáže is from the perfective verb ukázat.

Very roughly:

• ukazovat / ukazuje = showing, demonstrating, presenting, ongoing or repeated action
• ukázat / ukáže = to show once, to complete the showing

In a sentence about instruction or demonstration, ukazuje is very natural because it focuses on the process:

• Instruktor ukazuje správné držení těla.
  = The instructor is demonstrating correct posture.

If you used ukáže, it would sound more like:

• The instructor will show correct posture / will show it once or as a completed action

Because Czech has no articles.

English needs words like:

• a
• an
• the

Czech normally does not use them at all. So:

• Instruktor can mean an instructor or the instructor
• správné držení těla can mean correct posture or the correct posture, depending on context

Czech relies on:

• context
• word order
• emphasis
• shared knowledge

So you do not need to look for a Czech equivalent of the in this sentence.

Yes, Czech word order is more flexible than English because case endings show the grammatical roles.

The neutral order here is:

• Instruktor ukazuje správné držení těla.

This is a normal subject–verb–object order.

But other orders are possible for emphasis, for example:

• Správné držení těla ukazuje instruktor.
  = It is the instructor who is showing correct posture.
• Správné držení těla instruktor ukazuje.
  = Correct posture is what the instructor is showing.

So the original sentence is the most straightforward, but not the only possible order.

Because it is in the nominative case and the verb agrees with it.

Here:

• Instruktor = nominative singular masculine animate
• ukazuje = 3rd person singular

That tells you Instruktor is the one doing the action.

Meanwhile:

• správné držení těla is the object, what is being shown

In English, word order does most of that work. In Czech, case endings are very important.

Yes. Instruktor is a masculine animate noun.

That matters because masculine animate nouns have their own patterns, especially in the accusative and in plural forms.

In this sentence, Instruktor is singular nominative, so the form is just:

• instruktor

If you changed the sentence, you might see other forms:

• Vidím instruktora. = I see the instructor.
  Here it becomes instruktora in the accusative because it is masculine animate.

So it is useful to learn instruktor as:

• masculine animate
• nominative singular: instruktor

It is a noun, but it has a strong connection to a verb.

Držení is a verbal noun related to držet. In English, this is similar to nouns like:

• holding
• movement
• reading

So grammatically, držení behaves like a noun:

• it has gender: neuter
• it has case
• it can be described by an adjective: správné držení

But semantically, it still carries the idea of an action or manner of holding.

That is why držení těla literally feels like the holding of the body, even though in normal English we would usually just say posture.

Těla is pronounced approximately like tyela in slow, learner-friendly terms.

A few points:

• tě is not pronounced exactly like plain te
• ě often causes the consonant before it to sound softer or to gain a y-like glide
• so těla sounds roughly like tyeh-la

A careful approximation for English speakers is:

• TYEH-lah

But the best approach is to listen to native audio, because ě does not map perfectly onto English sounds.

Sometimes, but držení těla is the standard and very natural phrase for posture.

Depending on context, you might hear:

• postoj = stance, attitude, position
• pozice = position
• póza = pose

But these are not the same as posture in the general body-alignment sense.

So if you mean:

• correct way of holding the body
• good posture

then správné držení těla is exactly the right expression.