Wij leggen de proefperiode schriftelijk vast.

Because vastleggen is a separable verb in Dutch.

• The full verb is vastleggen.
• In a main clause, the conjugated part goes in the usual verb position, and the prefix moves to the end:
  • Wij leggen ... vast.
• In an infinitive or many subordinate clauses, it stays together:
  • Wij willen de proefperiode schriftelijk vastleggen.
  • ... omdat wij de proefperiode schriftelijk vastleggen.

This is very common in Dutch and often surprises English speakers.

Vastleggen often means:

• to put in writing
• to record
• to formally agree and document
• to lay down / stipulate

In this sentence, it suggests something a bit more official than just say or agree. It means the trial period is being made formal, usually in a written document.

It is leggen because the subject is wij (we), so the verb is plural:

• ik leg
• jij legt
• hij/zij legt
• wij leggen

And because vastleggen is separable, the conjugated part is leggen, while vast is moved to the end in a main clause.

So:

• Wij leggen ... vast = correct

Both wij and we mean we.

The difference is mostly about style and emphasis:

• we is more common in everyday speech
• wij can sound a bit more formal, careful, or emphasized

In business, legal, or written contexts, wij is very normal. Since this sentence sounds formal or contractual, wij fits well.

Proefperiode means trial period or probation period, depending on context.

It takes de because it is a de-word in Dutch:

• de proefperiode

So here, de proefperiode means the trial/probation period.

Here it functions as an adverb.

It describes how the action is done:

• schriftelijk vastleggen = to put in writing / to record in writing

Compare:

• een schriftelijke overeenkomst = a written agreement
  
  • here schriftelijke is an adjective
• Wij leggen het schriftelijk vast
  
  • here schriftelijk is an adverb

So in your sentence, it means something like in writing or in written form.

Because schriftelijk belongs with the verb phrase and comes before the separated particle vast.

The basic pattern is:

• subject
• finite verb
• object
• adverb(s)
• separable particle

So:

• Wij | leggen | de proefperiode | schriftelijk | vast

This word order is very natural in Dutch.

Yes, but only within normal Dutch grammar rules.

For example:

• Wij leggen de proefperiode schriftelijk vast.
  Neutral word order.
• De proefperiode leggen wij schriftelijk vast.
  The object is emphasized.
• Schriftelijk leggen wij de proefperiode vast.
  Schriftelijk is emphasized.

But vast will still usually stay at the end in a main clause, because it is the separated part of vastleggen.

Yes, they would understand it, but it sounds less natural than:

• Wij leggen de proefperiode schriftelijk vast.

Dutch usually places the direct object de proefperiode before the adverb schriftelijk in a neutral sentence. So the original sentence is the most standard version.

Yes, it often does.

It is common in:

• contracts
• HR language
• company policies
• official communication

It suggests that something is not just discussed verbally, but formally documented. So the tone is more formal than plain everyday speech.

In a subordinate clause, separable verbs usually become one word again:

• Wij leggen de proefperiode schriftelijk vast.
  main clause
• ... dat wij de proefperiode schriftelijk vastleggen.
  subordinate clause

This is an important pattern in Dutch:

• main clause: split
• subordinate clause / infinitive: together

Not especially; it is just the subject in first position, which is very normal.

Dutch main clauses follow the V2 rule, meaning the finite verb comes second. Since Wij is first, leggen must come second:

• Wij leggen ... vast.

If another element came first, the verb would still stay in second position:

• Vandaag leggen wij de proefperiode schriftelijk vast.
• In het contract leggen wij de proefperiode schriftelijk vast.

So the sentence is using the most straightforward, standard order.