input: Write a function to replace a given string with another string. The function should take three parameters: the original sentence, the word to be replaced, and the new word. It should return the modified sentence.
  assert replace_with_new("I love walking on the beach", "walking", "swimming") == "I love swimming on the beach"
  assert replace_with_new("Python is great", "great", "awesome") == "Python is awesome"
  assert replace_with_new("Sunny day at the park", "Sunny", "Rainy") == "Rainy day at the park"
  assert replace_with_new("", "old", "new") == ""
  assert replace_with_new("Working on my project", "project", "homework") == "Working on my homework"
  assert replace_with_new("Hello world!", "world", "Python") == "Hello Python!"
output: def replace_with_new(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)