input: Write a function to extract all verbs from a given sentence using the Natural Language Toolkit (NLTK). Return the verbs as a list.
  import nltk
  
  def extract_verbs(sentence):
      tokens = nltk.word_tokenize(sentence)
      tags = nltk.pos_tag(tokens)
      verbs = [word for word, pos in tags if pos.startswith('V')]
      return verbs
  
  assert extract_verbs("The cat jumped over the moon.") == ['jumped']
  assert extract_verbs("Run quickly and try to catch the bus.") == ['Run', 'catch']
  assert extract_verbs("He is reading a book.") == ['is', 'reading']
  assert extract_verbs("She went to the market.") == ['went']
  assert extract_verbs("") == []
output: import nltk
  
  def extract_verbs(sentence):
      tokens = nltk.word_tokenize(sentence)
      tags = nltk.pos_tag(tokens)
      verbs = [word for word, pos in tags if pos.startswith('V')]
      return verbs