Not all Sidhaisin idioms are readily translated into human equivalents, but here are some of the more easily understood ones.
Weather conditions and similar phenomena are described using the structure Ma va "It does". For instance, the English statement, "It is foggy" becomes in Sidhaisin Ma va iplai "It does fog". The adjective iplas "foggy" (along with other Sidhaisin weather-related modifiers) would never never used for actual weather conditions; rather, they are used in comparisons and metaphoric constructions to evoke the image of the appropriate weather.
Ad aute. The English phrase "...in nature" (as in "easygoing in nature") has a near-exact equivalent in the Sidhaisin ad aute "of soul", which is used in manner identical to the English.
Ma quindas iela'tra se. "That roots badly with me." Approximately equal to "I don't like it", "It doesn't impress or appeal to me", or "I don't hold with that", depending on the context in which it is found.
Tha maudra naithanel luth lival! Literally, "He has feathers for ears!" Equivalent to "He's crazy!" or "He's touched in the head!" -- in a humorous/friendly sense. For a more serious "that guy's insane" idiom, see below.
S'aila Sidhe. S'aila Rinua. Racial identifications do not use the adjective form, but rather the noun with an implied article: "I am elf. I am human."
Kienas! "Hairy!" Elven epithet for the ill-mannered or rude. About equal to "Animal!", "Barbarian!" or "Uncivilized!"
Fal iraisti o evol ad o shauthaa dru isosa. "Don't seek to borrow the dragon's eggs." Don't go looking for trouble. Often abbreviated as "o evol ad shauthaa" (or even just "shauthaa"), often with a sad shake of the head, to indicate someone or something that is a lightning rod for problems.
Tha o'thanye thienera. Literally, "he threw the leaf." He died.
Fal ha sha dru talara vo fal luth fitis essa. "You should not cross a river when looking for water." A common quote from the Tao-like philosophy of Ostysidhie, used to indicate futile, useless or wasted efforts.
Ya, lalas! "Oh, seventeen!". An expression of surprise or shock. Sometimes used in the sense of "I am surprised to hear that!"
Thas rethel sothera os o'thlai enelas. "His branches grew away from the sun." An expression indicating that the subject is insane or at the very least disturbingly abnormal.
Tha ha ledrith maudra beth thas liva. "He has a fox behind his ear." Used variously as "he's a tricky one", "he has something up his sleeve", or "don't trust him."
Ma utiasa levannethas ishel d'o gesol aila. "Now it's dangerous legs in the potatoes." An expression that is used when there is a sudden or unexpected change in the current situation.
This is only a small sampling of the wide range of Sidhas idiom. More will no doubt be uncovered and added to this page in the future.