I'm just a beginner of Spanish. An exercise of my Spanish text book asks the readers to ask questions about the bold part in the sentence:
Todo lo hacen muy rápido.
My answer is:
¿Cómo todo lo hacen?
The answer gives:
¿Cómo hacen todo?
I know that the "lo" in the original sentence refers to "todo", but why the "lo" is missing in the answer? I don't know when should we use assusatival pronouns to refer to the noun repeatedly. Is there a general rule?
It's not easy for me (native speaker) to distill the general rule. But, as a matter of fact:
¿Cómo hacen todo?
and
¿Cómo lo hacen?
would be correct constructions, though they would be slightly different questions. In the later, "lo", of course alludes to the thing that is being done ("todo" - direct object - you can try replacing it by "el trabajo"), which here is left tacit; hence, this would only make sense if the previous sentence or the context had mentioned that, eg: "-Siempre hacen todo de la misma manera. -¿Cómo lo hacen?"
So, your book/teacher is right.
To say
¿Cómo lo hacen todo?
would be a redundant construction (see that it's a merge of the two above). Incorrect, but it might happen in casual speaking, perhaps for some emphasis.
¿Como todo lo hacen?
would be definitely wrong, that's never the order.
"¿Cómo lo hacen todo?" would be as valid a question for "Todo lo hacen muy rápido" as the one given. Word of native speaker.
Your suggestion "¿Cómo todo lo hacen?" would not be in the correct order.
