input: Write a python function to classify a sentence into positive or negative.
  assert classify_sentence("I am very happy with the service") == "positive"
  assert classify_sentence("This is a terrible day") == "negative"
  assert classify_sentence("The product is okay, not too bad") == "neutral"
output: def classify_sentence(sentence):
      positive_keywords = ['happy', 'good', 'great', 'fantastic', 'excellent', 'love']
      negative_keywords = ['terrible', 'bad', 'worst', 'hate', 'awful']
      
      sentence_lower = sentence.lower()
      
      for word in positive_keywords:
          if word in sentence_lower:
              return "positive"
      
      for word in negative_keywords:
          if word in sentence_lower:
              return "negative"
      
      return "neutral"