input: Write a function to extract all the verbs from a given sentence. The function should take a sentence as input and return a list of verbs found in the sentence. If no verbs are found, the function should 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 running fast.")==['is', 'running']
  assert extract_verbs("This is a test.")==['is']
  assert extract_verbs("Where are you going?")==['are', 'going']
  assert extract_verbs("Hello, world!")==[]
output: 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