In april regent het vaak.

In Dutch, the names of months and days are normally not capitalized unless they begin a sentence. So:

• In april regent het vaak.
• Maandag is het druk.

This is different from English, where April and Monday are capitalized.

Regent is the present tense form of the verb regenen, which means to rain.

The verb is conjugated like this:

• het regent = it rains / it is raining

In this sentence, regent expresses something that happens often or typically, so English might translate it as it often rains.

Yes and no. In weather expressions, Dutch usually uses het as a dummy subject, just like English uses it in it rains.

So:

• Het regent. = It rains.

The het does not refer to a specific thing. It is just required by the grammar of the sentence.

Dutch main clauses normally follow the verb-second rule. That means the finite verb must come in the second position.

Here, In april has been placed first. Because that first slot is already taken, the verb regent must come next:

• In april | regent | het | vaak

So In april het regent vaak is incorrect in standard Dutch.

Yes, absolutely. That is also correct Dutch.

The difference is mainly one of focus:

• In april regent het vaak.
  This emphasizes in April.
• Het regent vaak in april.
  This starts with the basic statement it often rains and adds in April afterward.

Both are natural.

Dutch uses in with months, just like English:

• in april
• in mei
• in december

So if you want to say something happens during a month, in is the normal preposition.

Vaak means often.

In this sentence, it comes near the end:

• In april regent het vaak.

That is a very natural position for an adverb of frequency in Dutch. You will also see similar placement in other sentences, although adverb position can vary depending on emphasis and sentence structure.

It is in the present tense.

Dutch often uses the present tense for:

• general truths
• habits
• things that happen regularly

So In april regent het vaak means something like In April, it often rains as a general fact or usual pattern, not necessarily that it is raining right now.

Mostly yes. Dutch and English are quite similar here:

• Het regent. = It is raining / It rains.

One important similarity is that both languages use a dummy subject:

• Dutch: het
• English: it

So for an English speaker, this part is fairly familiar.