input: Write a python function to parse a sentence and extract its subject and predicate.
  assert parse_sentence("The cat sits on the mat.") == ('cat', 'sits')
  assert parse_sentence("John runs every morning.") == ('John', 'runs')
  assert parse_sentence("The birds fly high.") == ('birds', 'fly')
output: def parse_sentence(sentence):
      import nltk
      from nltk.tokenize import word_tokenize
      from nltk.tag import pos_tag
  
      nltk.download('averaged_perceptron_tagger')
      nltk.download('punkt')
  
      words = word_tokenize(sentence)
      tagged = pos_tag(words)
  
      subject = None
      predicate = None
  
      for i, (word, tag) in enumerate(tagged):
          if tag in ['NN', 'NNS', 'NNP', 'NNPS'] and not subject:
              subject = word
          elif tag.startswith('VB') and not predicate:
              predicate = word
          if subject and predicate:
              break
  
      return subject, predicate