Do I have to use the preposition "on" after "focus"?
Native speaker’s answer
Rebecca
For this particular sentence, yes you do. The word "focus" is fine by itself in a sentence and has meaning. However, if you take the word "on" out of the sentence, it becomes "Focus one thing, not everything." This sentence does not make sense in English. You would be taking away the important meaning out of this sentence if you removed the word "on."