I know both "He is legally dead" and "He is dead legally" mean the same, and I know both 'legally' are adverbs.

But one thing I don't understand: Why we say "It is raining heavily" and not "It is heavily raining"?

I know the latter one is wrong, I just don't know why. Can anyone answer me?

And can anyone please tell me what is the word 'beautifully' in 'A beautifully decorated house' called in grammar? Adverb or adjective?

Thanks a lot.
Adverbs, when modifying adjectives, are normally used in preposition. There are some exceptions to this rule, however. Some adverbs such as galore and enough are always used in postposition.

However, with verbs, adverbs of manner are always used postpositively.

"Beautifully" is an adverb.
I don't know that I agree that "He is dead legally" is okay at all!

In general, adverbs that modify verbs go after the verb.

You played well.

He answered quickly.

She sang beautifully.

I did poorly.

It rained heavily.

In general, adverbs that are helping to modify nouns go before.

It was a well-played game.

She led us into a beautifully decorated room.

It was a beautifully written composition.

It was a poorly executued plan.

In general, when they modify predicate adjectives, it also goes before.

I am very happy.

He is legally dead.

She is extremely beautiful.

I suppose you can sum this up to say "before adjectives" and "after verbs" -- in general! There will always be exceptions.
Students: Are you brave enough to let our tutors analyse your pronunciation?
Thanks a lot. But...umm..what's postpositions?

One more question: Are "The room is beautifully decorated" and "The room is decorated beautifully" grammatically correct?

Post - after

Pre - before