As for negative examples, Munteanu and Marcu (2005) randomly paired sentences from their parallel data using two constraints: a length ratio not greater than two, and a coverage constraint that considers a negative example only if more than half of the words of the source sentence has a translation in the given target sentence according to some bilingual lexicon.
from a large parallel corpus, one can easily retrieve another target sentence, almost identical, containing most of the words that the true target sentence also contains.