For this reason the use of shock tactics to start off with can be helpful. The students' embarrassment at their poor results will act as a spur to further study and help them to overcome their initial complacency. All phrasal verbs are idioms and all idioms are phrasal verbs. Idioms are a separate part of the language you can choose either to use or omit. Idioms can be defined as colloquial expressions.
Idioms are fixed expressions that cannnot be changed.
Words with grammatical functions can be used idiomatically. All proverbs are idioms. All proverbs are sayings. You are more likely to find idioms in quality newspapers than in the tabloids or popular press. All idioms, by definition, are cliches.