An idiom can be a phrase or complete sentence whose meaning is not obvious from the individual words. We commonly do not guess what they are just by simply hearing them, they can be cut within the sentence provided, but there must be previous background and certain knowledge of how the idiom is used and the actual appropriateness.
All of the idioms directly remain in our speech as part of our culture. Henceforth it´s strongly important to know more about how to use them. The list of idioms, according to each language, is really long due to they merge from unexpected situations that make you think of ignoring any grammatical structure. However, they are not absolved from carrying all sort of phrase such as Noun Phrase, Adjectival Phrase, Adverbial Phrase, Prepositional Phrase and Verbal Phrase.
Idioms, anyway, have developed structures and their own formulae. I will show a presentation in which you´ll find them all with specific patterns. This presentation was possible by Orlando Trosell and thanks to my passionate interest in offering good material. This happened when I was studying for a TOELF, but i had to quit it due to i was feeling unwell. Good luck.
Eduardo Becerrit 