title: replace letters in python string
body: <p>Im writing a program for french that turns present tense verbs into past tense. The problem is that I need to replace letters but they are user inputed so I have to have it replacing the letters from the end of the line. Here's what I have so far, but it doesn't change the letters it just gives an error:</p>
  
  <pre><code>word = raw_input("what words do you want to turn into past tense?")
  word2= word
  
  if word2.endswith("re"):
   word3 = word2.replace('u', 're')
   print word3
  
  elif word2.endswith("ir"):
   word2[-2:] = "i"
   print word2
  
  elif word2.endswith("er"):
   word2[-2:] = "e"
   print word2
  
  else:
   print "nope"
  </code></pre>
  
  <p>I tried word replace and that doesn't work either, it just gives me back the same string. If some one could just give me an example and maybe explain it a little that would be awesome. :/</p>