input: Write a function to extract the verbs from a given sentence using NLTK. If the given sentence is empty then return an empty list.
  
  import nltk
  
  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
  
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She has been running since morning.")==['has', 'been', 'running']
  assert extract_verbs("The weather is nice today.")==['is']
  assert extract_verbs("")==[]
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