The Grammarian role is sometimes folded into the Wordmaster role. The role is to listen to incorrect or otherwise problematic use of the English language. This can be easy, but it requires careful listening.
The Grammarian also listens for especially GOOD or effective use of the language: distinctive phrases, unusual but appropriate formulations, and so on.
Unlike forgetting to use the WOTD, there are no fines for incorrect language usage (or rewards for especially good usage!)
Particular things to listen for include:
Incorrect use of a plural when singular should be used.
Mixing past and present tenses
Words that come in an awkward order, or that differ from what the speaker was apparently trying to say.
Please keep in mind that English many not be the speaker's native language, and that pointing our errors should be done gently, and in the spirit of helping.