Can I use stop/keep/deter even if the action is already happening?

For example, if a guy is helping terrorists currently, can I say "we need to stop/keep/deter him from helping terrorists"?

I checked dictionaries but I am not sure if the verbs can be used when the action is already happening.

I asked this question, about Prevent: Usage of a verb, Prevent

And I got a comment that prevent cannot be used if the action is happening. But according to dictionaries, prevent means stop, so I am confusing if I can use stop in the above situation. And as far as I know, stop, deter and keep mean the same, so I would like to ask those three verbs can be used even though the action is already happening.


It is indeed true that you can't prevent something that is already happening. It is also true that you can't stop or keep something from happening that is already happening.

Now, consider these sentences:

We need to stop Rover from eating too much.
We need to prevent Rover from eating too much.
We need to keep Rover from eating too much.
We need to deter Rover from eating too much.

If Rover has already eaten too much, we can still say any of these things, because it is obvious that we mean that we want to stop Rover from eating too much again.

On the other hand, if whatever you want to prevent/stop/etc. is a one-time occurrence, then once it has occurred, you can't prevent it. So, if you want to prevent your brother from finding out about the surprise birthday party, once anyone tells him about it, you can't prevent/stop/keep him from finding out about it any more.

I think that this distinction is where your confusion is coming from.

Finally, deter has a bit of a different meaning from the others. While the others have the idea of ensuring that something doesn't happen, deter has the idea of making it more difficult to happen. So, for example, locks deter thieves, but they don't prevent them from thievery. What is locked up is harder to steal, but not impossible.

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