input: Write a function to extract only the verbs from a given sentence. If the sentence is empty then return an empty list.
  assert extract_verbs("The cat jumped over the moon.") == ['jumped']
  assert extract_verbs("She runs fast and sings beautifully.") == ['runs', 'sings']
  assert extract_verbs("") == []
  assert extract_verbs("The sun rises in the east.") == ['rises']
  assert extract_verbs("He is reading a book.") == ['is', 'reading']
output: def extract_verbs(sentence):
      from nltk import pos_tag, word_tokenize
      if not sentence:
          return []
      words = word_tokenize(sentence)
      tagged = pos_tag(words)
      verbs = [word for word, pos in tagged if pos.startswith('VB')]
      return verbs