On your desk (or desktop) you have a dictionary and next to it you have (don’t admit it if you don’t) a style guide. Knowing the definition of a word is one thing; knowing how to use two similar words, or punctuate phrase and clauses, or build more sophisticated sentences accurately is very much another. An excellent modern style guide is Garner’s Modern American Usage by Bryan Garner (4th edition, Oxford, 2016); older ones, such as A Dictionary of Contemporary American Usage by Bergen and Cornelia Evans (Random House, 1957), can be quite instructive as well.
One topic probably every style guide takes up is how to use the word like. Traditional standards of English define like principally as a preposition, not a conjunction, so a sentence such as like the doctor explained, you should eat something before taking that medicine should be corrected to as the doctor explained. A preposition always takes a noun or pronoun as an object; a conjunction introduces a clause. To write like the doctor explained is to push a clause (doctor explained) into the position of a prepositional object, and therein lies the traditional problem. Exchange the conjunction as for the preposition like and we’re all back on the straight and narrow.
So how can we better understand the complaint that style guides make against the phrase like when? Take this casual conversation, for example: You don’t know what bungee jumping is? It’s like when you lose your mind, tie yourself to an elastic cord, and jump off a bridge. If the word like should best be used as a preposition, what then is its object here? We would have to answer, the three following subordinate clauses (the last two of which have omitted the words when you): when you lose, when you tie, when you jump. But if a conjunction, not a preposition, should introduce a clause, can we really just again replace like with as: It’s like as you lose your mind? We cannot.
What’s going on here is actually an interesting little grammatical set piece. A clause is a group of words with a subject and verb, and clauses are classified into independent and subordinate; independent clauses assert a complete thought on their own and subordinate clauses do not. Subordinate clauses, in turn, are classified into noun clauses, adjective clauses, and adverbial clauses, which is determined in part by the word with which each begins. The subordinate clause when you lose your mind begins with the adverb when, and so that section of the sentence is properly an adverbial clause, as properly used, for example, in the sentence I’m always worried when you lose your mind and jump off a bridge. In our original sentence, though, the adverbial clause when you lose your mind is being employed as a noun clause, because the preposition like requires a noun (or, more properly, what is called a substantive) as its object. And therein, once again, the problem lies.
So what to do? In a casual conversation like the one we’ve imagined here, nothing at all. Relax and enjoy how playful language can be. But if we wish to suggest in such a conversation that we regard this whole matter of jumping off bridges of more serious concern, our language needs to change to reflect that. We could, for example, transform the adverbial-cum-noun clause when you lose your mind into a gerund: You don’t know what bungee jumping is? It’s like losing your mind, tying yourself to an elastic cord, and jumping off a bridge. A gerund is a noun built from a verb, and so now we have given the preposition like three proper noun objects: losing, tying, and jumping. And if our reaction turns even more earnest, we could dispense with the simile (like), and transform the statement into a powerful metaphor: it’s losing your mind. What was originally being compared now becomes fully identified.
And perhaps that’s the best choice, for reasons both grammatical and perilous.