All languages have idioms, defined in Webster’s ‘as an expression established in the usage of a language that is peculiar to itself either in grammatical construction (as no, it wasn’t me) or in having a meaning that cannot be derived as a whole from the conjoined meanings of its elements (as Monday week for “the Monday a week after next Monday”; many a for “many taken distributively”; had better for “might better”; how are you? for “what is the state of your health or feelings?”).’ In learning a language, speakers have to learn not just the rules of grammar but also the rules defining idiomatic use of the language. This happens naturally in the case of native speakers acquiring their own language from childhood on; and more or less naturally in the acquisition of a foreign language as well.
Idioms involve a certain degree of arbitrariness with respect to their incorporation of a language’s collocation rules, and a non-native speaker has to navigate these linguistic shoals in learning to distinguish between idiomaticity and normative grammar. Here is an example of the difficulty involved as overheard in an interview broadcast recently on the BBC World Service with the Belorussian ethno-jazz singer Rusia, who appears to have a fluent command of English. It has to do with the unidiomatic word *schoolguy concocted on the pattern of schoolboy and schoolgirl. Rusia uttered this word during the interview as part of the phrase “schoolguys and schoolgirls,” thereby violating the rules of idiomatic word formation in English, even as she followed the rules of compounding. It just so happens that English has only schoolboy but no *schoolguy. Hence Rusia could be said to have failed to observe the boundary between freedom and constraint inhering in the rules of English word formation.
By contrast, a seven-year old boy with English as a first language already “knows” that he has the freedom to coin the phrase word blind on the model of color blind, when he utters it in reference to his golden retriever in the process of asserting to his ten-year-old sister that the their pet dog is not “word-blind” qua dog despite being unable to read. Cf. Psalms 8:2 “Out of the mouth of babes and sucklings hast thou ordained strength . . .,” etc.