input: Write a function to extract and return the verbs from a given sentence using the nltk library. If the sentence is empty, return an empty list. 
  from nltk import word_tokenize, pos_tag
  
  def extract_verbs(sentence):
      if not sentence:
          return []
      tokens = word_tokenize(sentence)
      tags = pos_tag(tokens)
      return [word for word, pos in tags if pos.startswith('V')]
  
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She runs fast.")==['runs', 'fast']
  assert extract_verbs("He eats apples.")==['eats']
  assert extract_verbs("I will write a program.")==['will', 'write']
  assert extract_verbs("")==[]
output: from nltk import word_tokenize, pos_tag
  
  def extract_verbs(sentence):
      if not sentence:
          return []
      tokens = word_tokenize(sentence)
      tags = pos_tag(tokens)
      return [word for word, pos in tags if pos.startswith('V')]