What are the rules when extracting SVO triples from preprocessed text?
The goal of Subject-Verb-Object (SVO) triples is to extract a single triple for a sentence.
The sentence:

A rare black squirrel has become a regular visitor to a suburban garden.

results in the following SVO:

(squirrel, become, visitor)

Triplet Extraction From Sentences by Rusu et al. outlines how to do that. First, you'll need a parse tree ( Stanford Parser and OpenNLP are the most common). The three items then can be extracted:

The subject will be found by performing breadth first search and selecting the first descendent of NP that is a noun.


… the predicate of the sentence, a search will be performed in the VP subtree. The deepest verb descendent of the verb phrase will give the second element of the triplet.


… we look for objects. These can be found in three different subtrees, all siblings of the VP subtree containing the predicate. The subtrees are: PP (prepositional phrase), NP and ADJP (adjective phrase). In NP and PP we search for the first noun, while in ADJP we find the first adjective.