Kiristä ruuvi ennen kuin testaat jarrun.

Kiristä is the second-person singular imperative of kiristää = to tighten.

So Kiristä! means Tighten! or Tighten it!

In this sentence, it is a command directed at one person:

• Kiristä ruuvi = Tighten the screw

Finnish usually does not use the pronoun sinä in a command unless you want extra emphasis.

The verb form already shows that the command is aimed at one person:

• kiristä = tighten! (to one person)
• testaat = you test / you will test

So the sentence naturally leaves sinä out.

This is a very common Finnish object-case question.

In an affirmative imperative sentence, a total object in the singular usually appears in the nominative form.

So:

• Kiristä ruuvi = Tighten the screw

Even though in a normal statement you would often get:

• Sinä kiristät ruuvin = You tighten the screw

So the imperative changes the object pattern here.

Ennen kuin means before when it introduces a whole clause.

Here it connects:

• Kiristä ruuvi = main clause
• ennen kuin testaat jarrun = subordinate clause

So:

• ennen kuin testaat jarrun = before you test the brake

Compare:

• ennen testiä = before the test
• ennen kuin testaat = before you test

So ennen kuin is used before a clause with a finite verb.

Finnish often uses the present tense for actions that are in the future when the time relationship is already clear from context.

Here ennen kuin already tells us the testing happens after the tightening:

• testaat literally looks like you test
• but in context it means before you test / before you will test

This is completely normal Finnish.

Here jarrun is the form used for a singular total object in a normal active clause:

• testaat jarrun = you test the brake as a complete, bounded action

This contrasts with:

• jarrua = a partial object, which can suggest an ongoing, incomplete, or less bounded action

So very roughly:

• testaat jarrun = you test the brake fully / you carry out the test
• testaat jarrua = you are testing the brake / trying the brake out

In many grammar explanations, jarrun is described as the genitive form of the object, though in object grammar you may also see discussion of the accusative. For a learner, the key point is: this sentence needs jarrun because the object is total.

Because the two verbs are in different kinds of clauses:

• kiristä is an imperative
• testaat is an indicative finite verb

In Finnish, the form of a total object depends partly on the verb form:

• with an affirmative imperative, singular total object → usually nominative
  
  • Kiristä ruuvi
• with an ordinary active finite clause, singular total object → usually genitive-shaped
  • testaat jarrun

This difference is very normal in Finnish, even though it feels strange from an English point of view.

Yes. Finnish word order is fairly flexible.

The given sentence is very natural:

• Kiristä ruuvi ennen kuin testaat jarrun.

But you could also say:

• Ennen kuin testaat jarrun, kiristä ruuvi.

That version puts more focus on the time condition first: Before you test the brake, tighten the screw.

Both are correct.

Finnish has no articles like English a/an and the.

So:

• ruuvi can mean a screw or the screw
• jarrun can mean a brake or the brake (in object form)

The exact meaning depends on context. In a practical instruction like this, English often uses the:

• Tighten the screw before you test the brake.

Grammatically, jarrun is singular, from jarru = brake.

Whether it means:

• one specific brake,
• the brake mechanism,
• or the braking function being tested

depends on context. Finnish often leaves that kind of detail to the situation. In technical instructions, a singular noun can still refer to the relevant part or function in a practical way.

The dictionary forms are:

• kiristää = to tighten
• ruuvi = screw
• ennen kuin = before
• testata = to test
• jarru = brake

The forms used in the sentence are:

• kiristä = imperative of kiristää
• ruuvi = nominative singular
• testaat = you test from testata
• jarrun = object form of jarru

These are the forms you would expect in a practical instruction.