Tegen de tijd dat de proefperiode begint, is de thuiswerkplek ergonomisch.

Tegen de tijd dat is a fixed expression meaning by the time that.

• tegen here does not mean literal against
• de tijd = the time
• dat introduces the clause that follows

So Tegen de tijd dat de proefperiode begint is a natural way to say By the time the trial/probation period starts.

A simpler alternative is often als or wanneer, but tegen de tijd dat specifically emphasizes that something will already be true at that point.

Because dat de proefperiode begint is a subordinate clause.

In Dutch subordinate clauses, the conjugated verb usually goes to the end:

• main clause: De proefperiode begint
• subordinate clause: dat de proefperiode begint

This is one of the most important Dutch word order rules to learn.

Compare:

• De les begint morgen.
• Ik weet dat de les morgen begint.

In both subordinate clauses, the verb moves to the end.

Because the sentence begins with a fronted time clause: Tegen de tijd dat de proefperiode begint.

When something other than the subject comes first in a Dutch main clause, Dutch uses inversion: the verb comes before the subject.

Normal main clause order:

• De thuiswerkplek is ergonomisch.

After a fronted element:

• Tegen de tijd dat de proefperiode begint, is de thuiswerkplek ergonomisch.

So this is the usual verb-second rule in Dutch:

1. first element: Tegen de tijd dat de proefperiode begint
2. second element: is
3. then the subject: de thuiswerkplek

Proefperiode is a noun meaning something like trial period or probationary period, depending on context.

It is a compound:

• proef = trial / test
• periode = period

Dutch makes compound nouns very freely, and they are usually written as one word. So proefperiode is one word, not two.

It is a de-word:

• de proefperiode

Plural:

• de proefperiodes

Yes, often you can, depending on context.

• proeftijd is very common for probation period, especially in work or legal contexts
• proefperiode is also understandable and often sounds a bit more general: a trial period, testing phase, introductory period, etc.

So if this is specifically about a new employee’s probation period, proeftijd may sound slightly more idiomatic in many contexts. But proefperiode is still perfectly understandable.

Because Dutch normally writes compound nouns as one word.

Here:

• thuis = at home
• werk = work
• plek = place / spot

Together: thuiswerkplek = home workspace or work-from-home workstation

This is very normal in Dutch. English often uses separate words where Dutch uses one compound.

Other examples:

• werkplek = workplace/workstation
• slaapkamerdeur = bedroom door
• fietsband = bicycle tire

Because thuiswerkplek is a de-word.

In Dutch, every singular noun is either a de-word or a het-word. Unfortunately, you often just have to learn this with the noun.

Here it is:

• de thuiswerkplek

Plural:

• de thuiswerkplekken

A useful tip: many nouns ending in -plek are de-words, because plek itself is a de-word:

• de plek
• de werkplek
• de parkeerplek
• de thuiswerkplek

Because ergonomisch is used here as a predicate adjective, after the verb is.

In Dutch, adjectives usually do not take -e when they come after a linking verb like zijn:

• De thuiswerkplek is ergonomisch.
• De stoel is comfortabel.
• Het bureau is groot.

But when the adjective comes before the noun, it usually does take -e:

• de ergonomische thuiswerkplek
• de comfortabele stoel
• het grote bureau

So:

• de thuiswerkplek is ergonomisch = predicate adjective, no -e
• de ergonomische thuiswerkplek = attributive adjective, usually with -e

Yes. That is correct, but it means something slightly different grammatically.

• De thuiswerkplek is ergonomisch.
  The home workspace is ergonomic.

• De ergonomische thuiswerkplek
  the ergonomic home workspace

The first is a full sentence.
The second is just a noun phrase.

So if you want a full sentence with that form, you could say:

• De ergonomische thuiswerkplek is klaar.

Here dat introduces the subordinate clause after Tegen de tijd.

So the structure is:

• Tegen de tijd = by the time
• dat de proefperiode begint = that the trial period begins

This dat is a conjunction, not the demonstrative that as in that one.

You will often see dat introducing subordinate clauses:

• Ik weet dat hij komt.
• Ze zegt dat het klopt.
• Tegen de tijd dat de proefperiode begint...

Because the sentence starts with a subordinate clause, and that clause is separated from the main clause by a comma.

Structure:

• subordinate clause: Tegen de tijd dat de proefperiode begint
• main clause: is de thuiswerkplek ergonomisch

Dutch uses commas in this kind of sentence much like English does.

Yes, that is grammatically correct, but the nuance is a bit different.

• Als de proefperiode begint... = When the trial period starts...
• Tegen de tijd dat de proefperiode begint... = By the time the trial period starts...

Tegen de tijd dat suggests that the workspace will already be ergonomic before that moment arrives, or at least no later than then.
So it emphasizes a deadline or completed preparation.

That is why Tegen de tijd dat is a better match if you want the idea of already in place by then.

Yes. Ergonomisch is a normal adjective in Dutch, and it behaves regularly.

Examples:

• een ergonomische stoel
• een ergonomisch toetsenbord
• De werkplek is ergonomisch.

One thing to notice is that adjectives ending in -isch often look similar to English:

• logisch
• praktisch
• fantastisch
• ergonomisch

So this is a useful pattern for English speakers to recognize.

The sentence can be broken down like this:

1. Tegen de tijd dat de proefperiode begint
   fronted time clause

2. is
   finite verb of the main clause

3. de thuiswerkplek
   subject

4. ergonomisch
   predicate adjective

So the pattern is:

[Subordinate clause], [finite verb] [subject] [rest]

This is a very common Dutch sentence pattern when you begin with a time phrase or another clause.