Final answer:
An unacceptable usage of 'like' or 'as' is evident in mixed sentence constructions and improper comparisons, such as using 'like' when 'as if' or 'as though' is needed, or 'is when' and 'just because... doesn't mean...' constructs. Proper use of 'like' or 'as' is essential for creating effective similes and maintaining a consistent tone in narration. Overuse or incorrect usage can result in a lapse in tone and clarity.
Step-by-step explanation:
An unacceptable usage of like/as occurs when these words are used improperly to create comparisons or when used in mixed sentence constructions that should be avoided in writing. For example, using 'like' when 'as if' or 'as though' is more appropriate can lead to unclear comparisons. The construct 'just because... doesn't mean...' is common in speech but can create an informal tone that is unsuitable for formal writing. Similarly, the use of 'is when' in a sentence is another example of mixed construction that reduces the clarity and precision of the writing.
Using like or as appropriately is crucial in similes, which are a figurative language tool for comparing two unlike things to add vividness or clarity to the description. For instance, saying 'Learning a foreign language is like learning to ride a bicycle' effectively uses 'like' to create a simile, comparing the process of learning a language to the process of learning to ride a bicycle, emphasizing the multitasking required in both tasks.
Finally, it's important to recognize when 'like' is used as a preposition in noun phrases that act as subject complements, which is generally acceptable, as in 'The car looked like a Model T', but this can sometimes be confused with simile constructions.