The He failed to completely understand the book example is not a very good one He failed to understand the book completely is not at all ambiguous it can only mean that the subjects understanding was incomplete  when the adverb occurs at the end of the sentence it really isnt possible to consider it as a modifier of the word failed alone unless the sentence is spoken out loud with unnaturally strong emphasis being placed on the word completely a Not true both interpretations for failed to understand  completely are available with broad focus intonation b Regardless of the effect of intonation intonation is not available in written language so the problem remains in that situation