title: Issues in lemmatization (nltk)
body: <p>I am using nltk lemmatizer as follows.</p>
  
  <pre><code>from nltk.stem import WordNetLemmatizer
  lemmatizer = WordNetLemmatizer()
  mystring = "the sand rock needed to be mixed and shaked well before using it for construction works"
  splits=mystring.split()
  mystring = " ".join(lemmatizer.lemmatize(w) for w in splits)
  print(mystring)
  </code></pre>
  
  <p>I am expecting the output to be</p>
  
  <pre><code>sand rock need to be mix and shake well before use it for construction work
  </code></pre>
  
  <p>However, in the output I get (mentioned below) it seems like words such as <code>needed, mixed, shaked, using</code> have not changed to its base form.</p>
  
  <pre><code>sand rock needed to be mixed and shaked well before using it for construction work
  </code></pre>
  
  <p>Is there a way to resolve this problem?</p>