input: Write a function to replace a given string with another string in a sentence. The function should take three parameters: the original sentence, the old word to be replaced, and the new word to replace with. It should return the new sentence after the replacement.
  
  
  def replace_string(sentence, old_word, new_word):
      new_sentence = sentence.replace(old_word, new_word)
      return new_sentence
  
  assert replace_string("I love walking on the beach", "walking", "swimming") == "I love swimming on the beach"
  assert replace_string("Reading is a great hobby", "Reading", "Writing") == "Writing is a great hobby"
  assert replace_string("Technology is evolving rapidly", "rapidly", "slowly") == "Technology is evolving slowly"
  assert replace_string("", "old", "new") == ""
  assert replace_string("Hello world!", "world", "universe") == "Hello universe!"
  
output: def replace_string(sentence, old_word, new_word):
      new_sentence = sentence.replace(old_word, new_word)
      return new_sentence