input: Write a function to extract the verbs from a given sentence using the NLTK library and print them. If the input sentence is empty, return an empty 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("She is singing a song.")==['is', 'singing']
  assert extract_verbs("")==[]
  assert extract_verbs("Running and swimming are fun activities.")==['Running', 'are', 'swimming']
  assert extract_verbs("I love to read books.")==['love', 'read']
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