input: Write a function to extract only the verbs from a given sentence. The function should return a list of verbs. If no verbs are found, it should return an empty list.
  assert extract_verbs("The cat jumped over the moon.")==["jumped"]
  assert extract_verbs("She quickly ran to the store.")==["quickly", "ran"]
  assert extract_verbs("I am happy.")==["am"]
  assert extract_verbs("The sun is shining.")==["is", "shining"]
  assert extract_verbs("Hello, world!")==[]
output: import nltk
  nltk.download('punkt')
  nltk.download('averaged_perceptron_tagger')
  
  def extract_verbs(sentence):
      words = nltk.word_tokenize(sentence)
      tagged = nltk.pos_tag(words)
      verbs = [word for word, pos in tagged if pos.startswith('VB')]
      return verbs