Nemojte da vežbate pre nego što se istegnete.

Because nemojte is the usual way to make a negative command in Serbian.

So instead of simply negating the verb, Serbian commonly uses:

nemoj / nemojte + da + present tense

In this sentence:

• nemojte = don’t when speaking to more than one person or to one person formally
• da vežbate = that you exercise / to exercise in this construction

So Nemojte da vežbate means Don’t exercise.

A few related forms:

• Nemoj da vežbaš. = Don’t exercise. (informal singular)
• Nemojte da vežbate. = Don’t exercise. (plural or formal singular)

After nemojte, Serbian normally uses da + present tense.

So:

• nemojte da vežbate
• literally something like don’t that you exercise
• naturally: don’t exercise

This is a very common Serbian pattern. English uses the bare verb after don’t, but Serbian usually uses da plus a present-tense verb.

This is one of the first command patterns learners need to get comfortable with:

• Nemojte da kasnite. = Don’t be late.
• Nemoj da pričaš. = Don’t talk.

Vežbati can mean several related things, depending on context:

• to exercise
• to practice
• to train

In this sentence, because it is followed by stretching, vežbate clearly means to exercise / work out.

Grammatically, vežbate is the 2nd person plural present form of vežbati.

It can refer to:

• you all exercise
• you exercise when addressing one person formally

Both.

Nemojte is used for:

• you all
• you when speaking politely/formally to one person

So this sentence could be said to:

• a group in a gym class
• one person in a respectful or formal setting

If you were talking to one friend informally, you would say:

• Nemoj da vežbaš pre nego što se istegneš.

Pre nego što means before when introducing a whole clause.

It is made of three parts:

• pre = before
• nego = than
• što = what / that

But you should learn pre nego što as a fixed expression meaning before + clause.

Here it connects two actions:

• vežbate = you exercise
• se istegnete = you stretch

So it means:

• before you stretch

A useful comparison:

• pre treninga = before the workout
  
  • before + noun
• pre nego što počnete = before you begin
  
  • before + clause

Because the verb is istegnuti se or istezati se, a reflexive verb meaning to stretch oneself / to stretch.

The little word se is very common in Serbian and often appears with verbs that are reflexive or behave like reflexive verbs.

So:

• istegnuti = to stretch something out
• istegnuti se = to stretch oneself

In this sentence, se istegnete means you stretch.

English often leaves out oneself, but Serbian keeps the reflexive marker.

The sentence uses the verb in a form based on the perfective verb istegnuti se.

So:

• istegnuti se = to stretch oneself out, to do the stretching
• istezati se = to be stretching, to stretch repeatedly/habitually

In advice or instructions like this, Serbian often uses the perfective form for a completed preparatory action:

• first stretch
• then exercise

So pre nego što se istegnete suggests before you have stretched / before you stretch as a complete action.

You may also hear forms from istezati se in other contexts, but this sentence is natural and standard.

Because in Serbian, after:

• nemojte da ...
• pre nego što ...

you often use present-tense forms, even though the overall meaning may be a command or may refer to a future action.

So the forms look present tense, but the meaning depends on the structure.

Here:

• Nemojte da vežbate = Don’t exercise
• pre nego što se istegnete = before you stretch

This is very normal in Serbian. English and Serbian do not match word-for-word in these clause patterns.

In standard Serbian, nemojte da + present tense is the normal and safest pattern.

So the standard form is:

• Nemojte da vežbate.

Learners should definitely use that pattern.

You may sometimes encounter shorter or different patterns in speech or in neighboring standards, but if you are learning Serbian, nemojte da + present is the one to remember.

The sentence is:

• Nemojte = don’t
• da vežbate = exercise
• pre nego što = before
• se istegnete = you stretch

So the structure is:

Negative command + main action + time clause

More literally:

Don’t exercise before you stretch.

The word order is very natural and neutral. Serbian is somewhat flexible with word order, but this version is the normal straightforward one.

You would change the verb forms from plural/formal to informal singular:

• Nemoj da vežbaš pre nego što se istegneš.

Changes:

• nemojte → nemoj
• vežbate → vežbaš
• istegnete → istegneš

So the meaning stays the same, but the person addressed changes.

Yes, basically.

The difference is mainly one of standard spelling/pronunciation:

• Serbian Ekavian: vežbati, vežbate
• Croatian / Bosnian / some Serbian varieties Ijekavian: vježbati, vježbate

The meaning is the same. In the sentence you were given, the spelling is standard Ekavian Serbian.

The main sounds English speakers often notice are:

• ž in vežbate = like the s in measure
• š in što = like sh in shoe
• č / ć are not in this sentence, but they are often confused with š and ž

A rough pronunciation guide:

• Nemojte ≈ neh-MOY-teh
• vežbate ≈ VEZH-bah-teh
• pre nego što ≈ preh NEH-go shtoh
• se istegnete ≈ seh ee-STEG-neh-teh

Not perfect phonetic notation, but good enough to help an English speaker get started.