Learn Danish: Dagen før havde det regnet meget. - The day before, it had rained a lot.

This is the V2 (verb-second) rule in action. Because the sentence starts with the time expression Dagen før, the verb havde must come in the second position, swapping places with the subject det.

Why does the sentence use havde regnet instead of the simple past tense regnede?

Havde regnet is the past perfect tense (had rained). It is used because the raining happened before whatever past event the speaker is mainly talking about. If you just said regnede, it would mean it rained at that exact time, rather than leading up to it.

What is the difference between using Dagen før and I går here?

I går strictly means yesterday (relative to today). Dagen før means the day before a specific event you are already discussing in the past. This makes Dagen før the right choice when telling a story about the past.

Why does regnet end in a -t?

This is the past participle form of the verb at regne (to rain). Whenever you build a compound tense like the past perfect using havde (or the present perfect with har), regular verbs get this -t ending.