Adverb placement is very important. However, it is often flexible, which may lead to ambiguity.
There is also the problem of adverbs and adjectives.
Happily I sent Johnny home. (I'd say we're safe to say this is an adverb.)
I happily sent Johnny home. (This is an adverb.)
Adverb placement of focusing adverbs generally takes place in the middle of a sentence, or in the 'mid-position'. Focusing adverbs put the emphasis on one part of the clause in order to modify, qualify or add additional information. Adverbs of frequency (sometimes, usually, never, etc.), adverbs of certainty (probably, certainly, etc.) and comment adverbs (adverbs expressing an opinion such as 'intelligently, expertly, [happily,] etc.') can all be used as focusing adverbs.
Sam stupidly left his computer at home.
I happily sent Johnny home.
I sent Johnny happily home. (This may be considered an adjective.)
The placement in the last case removes the ambiguity over who was happy.
You can also use an adjectival phrase to make a distinction.
I sent Johnny home a happy man.
I sent Johnny home to his delight.
Again, here it's clear who was happy.
Or you can restructure the sentence somewhat to make it clearer still:
I was glad to send Johnny home happily.
Here we know who is glad and who is happy.