Hvis dokumentet ikke er klart, sender min chef det tilbage.

Because dokumentet is the definite form of et dokument.

• et dokument = a document
• dokumentet = the document

In Danish, the definite article is usually added to the end of the noun as a suffix:

• en chef → chefen
• et dokument → dokumentet

So here, dokumentet means the document.

Because dokumentet is a neuter noun (et-word), and the adjective agrees with it.

• et dokument
• therefore: dokumentet er klart

Compare:

• en plan er klar
• et dokument er klart

So the -t is there because the thing being described is neuter singular.

Because this is a subordinate clause introduced by hvis.

In Danish subordinate clauses, sentence adverbs like ikke usually come before the finite verb:

• Hvis dokumentet ikke er klart ...

Structure:

• Hvis = if
• dokumentet = the subject
• ikke = not
• er = is
• klart = clear / ready

This is different from normal main-clause word order, where ikke often comes after the finite verb.

Because Danish uses V2 word order in main clauses: the finite verb must come in the second position.

The opening if-clause counts as the first element:

• Hvis dokumentet ikke er klart, = first element
• so the finite verb of the main clause must come next:
• sender
• then the subject:
• min chef

So:

• Hvis dokumentet ikke er klart, sender min chef det tilbage.

If you started directly with the subject, you would say:

• Min chef sender det tilbage, hvis dokumentet ikke er klart.

Both are fine, but the word order changes depending on what comes first.

Hvis means if in a conditional sense.

It introduces a condition:

• Hvis dokumentet ikke er klart ...
• If the document is not ready/clear ...

It is used when one thing depends on another.

A learner may compare it with om, but om is usually used more for whether, not this kind of condition.

For example:

• Jeg ved ikke, om dokumentet er klart. = I don't know whether the document is ready.
• Hvis dokumentet ikke er klart, ... = If the document isn't ready, ...

Det refers back to dokumentet.

Since dokument is a neuter noun (et dokument), the matching pronoun is det:

• et dokument → det

So:

• sender min chef det tilbage
• my boss sends it back

If the noun were a common gender noun (en-word), Danish would often use den instead.

Tilbage means back.

So sende noget tilbage means to send something back.

In this sentence:

• sender min chef det tilbage
• my boss sends it back

This is a very common combination in Danish:

• give tilbage = give back
• komme tilbage = come back
• gå tilbage = go back
• sende tilbage = send back

It can suggest either, depending on context.

In Danish, klar / klart can mean things like:

• clear
• ready
• finished enough
• prepared

With a document, the intended meaning is often something like:

• the document is not ready
• the document is not finalized
• the document is not clear enough

So the exact English wording depends on context, even though the Danish is natural in all those senses.

Because the sentence begins with a subordinate clause:

• Hvis dokumentet ikke er klart, ...

That clause is followed by the main clause:

• sender min chef det tilbage.

In standard Danish writing, it is normal to separate that opening clause from the main clause with a comma.

Yes. Sender is the present tense of sende.

But like English, Danish present tense can describe:

• something habitual
• something generally true
• something that will happen in the future, depending on the condition

So this sentence can mean something like:

• If the document isn't ready, my boss sends it back
  or more naturally in English:
• If the document isn't ready, my boss will send it back

Danish often uses the present tense where English might prefer will.

Yes. A very natural alternative is:

• Min chef sender det tilbage, hvis dokumentet ikke er klart.

This has the same basic meaning:

• My boss sends it back / will send it back if the document isn’t ready.

The difference is mainly in emphasis and structure:

• Starting with Hvis ... highlights the condition first.
• Starting with Min chef ... starts with the main action.

Both are good Danish, but the word order in the main clause must still follow Danish rules.