Lemmatization is the task of determining that two words have the same root, despite their surface differences. The words am, are, and is have the shared lemma be; the words dinner and dinners both have the lemma dinner. Lemmatizing each of these forms to the same lemma will let us find all mentions of words in Russian like Moscow. The lemmatized form of a sentence like He is reading detective stories would thus be He be read detective story.