"Aamir goes to play."
Translation:आमिर खेलने जाता है।
खेलना is the infinitive. खेलने is in the oblique-infinitive case. This is when the infinitive of the verb is used as the object of a postposition. Of course, you don't see a postposition in the sentence but it is implicit -आमिर खेलने (के लिए) जाता है। where the 'के लिए' corresponds to the preposition 'to' in the English sentence. Another commentator on these forums referred to them as 'ghost postpositions' and I quite like the term.