input: Write a function to extract only the verbs from a given sentence using the Natural Language Toolkit (NLTK). The function should return a list of verbs found in the sentence. If no verbs are found, the function should return an empty list. 
  assert extract_verbs("The cat jumped over the moon.")==['jumped']
  assert extract_verbs("I want to dance.")==['want', 'dance']
  assert extract_verbs("The weather is great.")==[]
  assert extract_verbs("Running and swimming are fun.")==['Running', 'swimming']
output: def extract_verbs(sentence):
      from nltk import pos_tag, word_tokenize
      words = word_tokenize(sentence)
      tagged = pos_tag(words)
      verbs = [word for word, pos in tagged if pos.startswith('V')]
      return verbs