How to identify the subject of a sentence?

As NLTK book (exercise 29) says, "One common way of defining the subject of a sentence S in English is as the noun phrase that is the child of S and the sibling of VP."

Look at tree example: indeed, "I" is the noun phrase that is the child of S that is the sibling of VP, while "elephant" is not.

English language has two voices: Active voice and passive voice. Lets take most used voice: Active voice.

It follows subject-verb-object model. To mark the subject, write a rule set with POS tags. Tag the sentence I[NOUN] shot[VERB] an elephant[NOUN]. If you see the first noun is subject, then there is a verb and then there is an object.

If you want to make it more complicated, a sentence- I shot an elephant with a gun. Here the prepositions or subordinate conjunctions like with, at, in can be given roles. Here the sentence will be tagged as I[NOUN] shot[VERB] an elephant[NOUN] with[IN] a gun[NOUN]. You can easily say that word with gets instrumentative role. You can build a rule based system to get role of every word in the sentence.

Also look at the patterns in passive voice and write rules for the same.

You can use Spacy.

Code

import spacy
nlp = spacy.load('en')
sent = "I shot an elephant"
doc=nlp(sent)

sub_toks = [tok for tok in doc if (tok.dep_ == "nsubj") ]

print(sub_toks)