<p>Mediators were standing by, prepared to intervene in the labor dispute even though both sides had refused earlier offers for assistance.</p>
<p>I got this question wrong... what are some other idioms that are hard to detect?</p>
<p>to answer the question it should be offers “of” assistance not “for” assitance</p>
<p>The idiom in the sentence is offers of assistance (and it is of). I don’t know the number but I am guessing there are thousands of idioms and it is hard to think of them as a native speaker except when you actually say them. Two common ones are “used to be” when referring to formerly was and “supposed to be” when referring to should have been or should be. Neither use nor suppose have anything near those meanings by themselves without the d added at the end of the word and the to be added. Many make a common error when writing those by not putting the d at the end of use or suppose because when you speak them you don’t hear the d. Other common ones are “take a walk,” “beat a path,” “go for broke,” “eat crow,” “drink like a fish,” “lend a hand,” “in hot water,” “I hear you” (for I agree with you), “hit the books,” “burn the midnight oil” (for working long into the night).</p>