input: Write a function to replace a given string with another string. The function should take the original sentence, the old word to be replaced, and the new word as parameters and return the new sentence with the old word replaced by the new word.
  
  assert replace_string("I love walking on the beach", "walking", "swimming")=="I love swimming on the beach"
  assert replace_string("The cat sat on the mat", "cat", "dog")=="The dog sat on the mat"
  assert replace_string("This is a test", "is", "was")=="Thwas was a test"
  assert replace_string("Hello world", "world", "Python")=="Hello Python"
  assert replace_string("", "old", "new")==""
output: def replace_string(sentence, old_word, new_word):
      return sentence.replace(old_word, new_word)