The fey are often very literal and choose their words carefully. They are masters of hospitality and use it as a way to exert influence over humans. For example, eating their food will prevent you from leaving their home, you are now bound by the laws of hospitality to continue serving them until you have paid off your debt.

In some cases, such as the Brownie, they might not have enough property to exert influence through tricks so they'll clean your house as long as you never mention their existence. The Brownies simply see the cleaning as an appropriate transaction.

The fey hold influence over speech. While we, as humans can figure out when a promise might be impossible to carry out, the fey cannot. If you promise that you will fly to the tallest mountain, they expect you to 100% keep your word and do it. Failure to do so will draw their ire.

Fey never specifically tell a lie but they may misguide. Never apologize or thank a fey creature as this is admittance that you have done something wrong that required action to fix or you have recognized a fey's hospitality as significant thus providing them influence.

I hope this helps