input: Write a function to extract verbs from a given sentence. The function should tokenize the sentence, tag each token with its part of speech, and return a list of verbs.
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("She is running very fast.")==['is', 'running']
  assert extract_verbs("They quickly decided to go.")==['decided', 'go']
  assert extract_verbs("He sang beautifully.")==['sang']
  assert extract_verbs("We have completed the task.")==['have', 'completed']
output: import nltk
  nltk.download('punkt')
  nltk.download('averaged_perceptron_tagger')
  
  def extract_verbs(sentence):
      tokens = nltk.word_tokenize(sentence)
      tagged = nltk.pos_tag(tokens)
      verbs = [word for word, pos in tagged if pos.startswith('VB')]
      return verbs