input: Write a function to analyze a given sentence and extract only the verbs from it. Return the list of extracted verbs. If no verbs are found, 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
  
  sentence = "The cat jumped over the moon."
  verbs = extract_verbs(sentence)
  assert verbs == ["jumped"]
  
  sentence = "She is reading a book."
  verbs = extract_verbs(sentence)
  assert verbs == ["is", "reading"]
  
  sentence = "He went to the store."
  verbs = extract_verbs(sentence)
  assert verbs == ["went"]
  
  sentence = "This is a test."
  verbs = extract_verbs(sentence)
  assert verbs == ["is"]
  
  sentence = "Nothing to extract here."
  verbs = extract_verbs(sentence)
  assert verbs == []
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