You could use either sentence, depending on the context. _______
You would use the first sentence -- "How many robbers went into the bank?" – when you are referring to a completed action.
In this scenario, the questioner would want to know about a series of events. Some of the actions would be completed from start to finish, some not. In these questions, the questioner is asking about all completed actions:
"¢ Where were you on the morning of June 26th? "¢ Did you see anything out-of-the-ordinary? "¢ You say you saw some men in masks going in to the bank. Did you assume they were robbers? "¢ OK, we'll call them robbers now. HOW MANY ROBBERS WENT into the bank? "¢ Did you see anyone come out of the bank? etc.
In the sentence above – "how many robbers went..." – the questioner is referring only to the fact that xxx # of robbers went into the bank. What was happening at that moment or in the background is not important. The only thing important in that sentence is the number of robbers in the complete action.
There are, however, situations in which you would use the sentence: "How many robbers were going into the bank?"
The questioner might be asking questions about what was happening at a certain time, about an action that had not been completed or that you would be describing just a part of:
"¢ Where were you on the morning of June 26th? "¢ Did you see anything out-of-the-ordinary? "¢ You say you saw some men in masks going in to the bank. Did you assume they were robbers? "¢ OK, we'll call them robbers now. How many robbers WERE GOING INTO THE BANK when you saw them? "¢ Did you see anyone come out of the bank? "¢ You saw the robbers? WHAT WERE THE ROBBERS DOING as they came out of the bank. etc.
In these sentences, the verb phrases with the capital letters -- where the robbers were going, and what the robbers were doing -- are related to other actions. In the fourth sentence, "going into the bank" is what the robbers were in the middle of doing when you SAW them. Whatever the robbers were doing, you didn't see them start doing it and you didn't see them finish doing it.
In the last sentence, "what were the robbers doing" describes an unfinished action that the robbers were in the middle of as they came out of the bank, or, as they were coming out of the bank.
The key word here is "unfinished." _______
As always, you have to know the context and the relationship of one verb to another to make the appropriate choice of verb tense.
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