Learn Hardcore Dutch: Het is echter te donker om in het park te lopen. - However, it is too dark to walk in the park.

Het here is an impersonal or dummy subject, similar to English it in it’s raining. It doesn’t refer to any noun; Dutch often requires het as a subject when making general statements about conditions (here: darkness).

What does echter mean, and why is it placed between the verb and the adjective?

Echter is a conjunctive adverb meaning however or though. In a main clause Dutch word order, adverbs like echter typically come right after the finite verb. So you get: Subject – Verb – Adverb – Complement, i.e. Het is echter te donker.

Why is the adjective donker not inflected (no -e ending)? I thought adjectives always took -e before nouns.

Adjective inflection depends on its role. Here donker is used predicatively (after the linking verb zijn), so it stays uninflected: Het is donker. Attributive adjectives (before a noun) would take -e, e.g. het donkere park.

What is the construction te… om… te, and how does it work here?

Te + adjective + om + infinitive clause expresses too + adjective + to + verb. Literally: te donker om in het park te lopen
= too dark to walk in the park
You always use om before the infinitive clause, and the main verb of that clause gets te.

Why don’t we use a subordinate clause with dat instead of the infinitive construction?

Dutch offers two patterns:
1) Te + adjective + om + infinitive = too… to…
2) Zo + adjective + dat + finite clause = so… that…
You wouldn’t say Het is te donker dat we in het park lopen. Instead, for so dark that… you’d do:
Het is zo donker dat we niet in het park kunnen lopen.
For too dark to…, you use te… om… te.

Could we say Het is echter te donker om te lopen in het park instead?

Grammatically yes, but less idiomatic. Dutch normally places the prepositional phrase inside the infinitive clause as om + PP + te + verb:
om in het park te lopen
Moving in het park after te lopen is understood but sounds awkward in everyday speech.

What change if we replace lopen with wandelen?

Lopen is a general verb for walking or going on foot. Wandelen specifically means to stroll or take a walk (more leisurely). So
te donker om in het park te wandelen
emphasizes that even a casual stroll is impossible due to the darkness.

Can we start the sentence with echter, like Echter is het te donker om in het park te lopen?

You can front echter for emphasis, but it’s more formal or literary and triggers inversion:
Echter is het te donker om in het park te lopen.
In everyday speech, it’s more natural to keep echter after the verb.