title: Why NLTK Lemmatizer can't lemmatize some plural words?
body: <p>I have tried to lemmatize a words from Quran Holy Book, but some words can't be lemmatized.</p>
  
  <p>here's my sentence:</p>
  
  <pre><code>sentence = "Then bring ten surahs like it that have been invented and call upon for assistance whomever you can besides Allah if you should be truthful"
  </code></pre>
  
  <p>that sentence is part of my txt dataset.
  as you can see, there's "surahs" which is a plural form of "surah".
  I've tried my codes:</p>
  
  <pre><code>def lemmatize(self, ayat):
      wordnet_lemmatizer = WordNetLemmatizer()
      result = []
  
      for i in xrange (len(ayat)):
          result.append(wordnet_lemmatizer.lemmatize(sentence[i],'v'))
      return result
  </code></pre>
  
  <p>which when I run and print, the result is like this:</p>
  
  <pre><code>['bring', 'ten', 'surahs', 'like', u'invent', 'call', 'upon', 'assistance', 'whomever', 'besides', 'Allah', 'truthful']
  </code></pre>
  
  <p>the 'surahs' isn't changed into 'surah'.</p>
  
  <p>anybody can tell why? thanks.</p>