For those who came in late, a "sentence adverb" is a word that modifies an entire sentence rather than just the verb or predicate. A sentence adverb communicates speaker attitudes about the proposition that the sentence denotes, or discourse information. So "fortunately" is a sentence adverb in the sentence "Fortunately, we found a source of fresh water on the island." Also note "however," the sentence adverb for discourse.
I couldn't find much information on the Internet about sentence adverbs. Here are the three most helpful links I found:
http://www-01.sil.org/linguistics/GlossaryOfLinguisticTerms/WhatIsASentenceAdverb.htm http://grammar.about.com/od/grammarfaq/f/sentadvqa.htm http://www.oxforddictionaries.com/definition/english/sentence-adverb
All of the links define sentence adverbs as sentence modifiers. But is this always true? Note the following:
a) The man fell from the tenth story, fortunately onto a pile of pillows.
b) Fortunately, the man fell from the tenth story onto a pile of pillows.
In sentence a), "fortunately" clearly conveys speaker attitude and has the same meaning as it does in sentence b). However, the sentences don't necessarily have synonymous readings.
It can be unfortunate that the man fell from the tenth story onto a pile of pillows, and at the same time fortunate that he landed on a pile of pillows (rather than the hard pavement, for example).
So, if "fortunately" in sentence a) can convey speaker attitude about the state of affairs denoted by the prepositional phrase "onto a pile of pillows," what should we call "fortunately" in this context if not a sentence adverb?
I am assuming that other languages have sentence adverbs, and that this question is therefore not off-topic.