input: Write a function to extract verbs from a given sentence and return them as a list. If the sentence is empty, return an empty list.
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She is running and laughing.")==['is', 'running', 'laughing']
  assert extract_verbs("")==[]
output: def extract_verbs(sentence):
      from nltk.tokenize import word_tokenize
      from nltk.tag import pos_tag
      if not sentence:
          return []
      words = word_tokenize(sentence)
      tagged_words = pos_tag(words)
      verbs = [word for word, pos in tagged_words if pos.startswith('VB')]
      return verbs