input: Write a function to extract all verbs from a given sentence using the Natural Language Toolkit (NLTK) library. The function should return a list of verbs. If the given sentence is empty then return an empty list.
  
  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("I am running very fast.") == ['am', 'running', 'fast']
  assert extract_verbs("") == []
  assert extract_verbs("We study Python daily.") == ['study', 'daily']
  assert extract_verbs("She eats an apple eagerly.") == ['eats', 'eagerly']
output: def extract_verbs(sentence):
      if not sentence:
          return []
      tokens = nltk.word_tokenize(sentence)
      tags = nltk.pos_tag(tokens)
      verbs = [word for word, pos in tags if pos.startswith('V')]
      return verbs