Sentence:  The <WSD>informant</WSD> learns his language by formal training and, more importantly, by constant exposure to its use. He cannot repeat to the linguist what he has never seen or heard. 
Meaning of the word marked with <WSD> tags:  A native speaker who acts as a linguistic reference for a language being studied. The informant demonstrates native pronunciation, provides grammaticality judgments regarding linguistic well-formedness, and may also explain cultural references and other important contextual information.