Learn Dutch: Het regent steeds.

English often uses the verb "keeps" to show that an action is repeating. Dutch does this by using a simple verb (regent) combined with the frequency adverb steeds (constantly / all the time). So, Het regent steeds literally means "It rains constantly", which is the most natural way to say "It keeps raining" in Dutch.

Why does the frequency adverb steeds come at the very end of this sentence?

In a standard Dutch main clause, adverbs of frequency come after the conjugated verb (regent). Because there are no objects or other extra details in this short sentence, the adverb simply falls at the very end.

Could I use altijd instead of steeds here?

Yes, you could say Het regent altijd ("It always rains"). However, steeds gives more of a feeling of repetition or continuation ("it keeps on raining" or "it rains time and time again"), whereas altijd is a more general "always".

Why do we need the word het here?

Just like in English, Dutch needs a grammatical subject even when talking about the weather. We use het as a "dummy" subject for this. You cannot just say Regent steeds; it must be Het regent steeds (It rains constantly).