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Please tell me if the following sentences are the same or not?
It is ten o’clock. They could have arrived now.
It is ten o’clock. They could have arrived by now.
Are they same? If not, then please tell me what the difference between them is.
Thank you.
First of all, it's unusual to use the word could in this context- it would mean that it is possible that they have already arrived, but you don't know for certain. If that's what you meant, both sentences are correct, but the second is more idiomatic.
If you meant that you expected them to arrive before now, then should is the correct word to use.
It is ten o’clock. They should have arrived now.
This means that you expect that they have arrived, but you don't know for certain whether they have arrived.
It is ten o’clock. They should have arrived by now.
This means that you expected them to have arrived, but you know that they have not yet arrived- you are expressing concern that things are not going according to plan.
I disagree with JavaLatte that "could" is unusual in this context. I have used both of these sentences on multiple occasions when dealing with people arriving on airplanes, etc, though the first sentence is more of a complaint).
I agree that "could" can denote a possibly, that one is unsure whether it has occurred or not(Definition 1). However, "could" can also denote a possible future act or course of action(Definition 2), or an alternative action or course of action that was not taken (Definition 3).
To answer the question you actually asked: they are not the same, though they are similar. 1. This means that "They could have arrived at 10 o'clock(the present time)". 2. This means that "They could have arrived at or before 10 o'clock(the present time)".
In addition to the bolded sections, if I was interpreting these sentences, I would think that the first sentence uses the third definition of "could", e.g. that the person had the ability to arrive at the current time, but didn't, either arriving earlier, or having not yet arrived. This would generally be used in a complaint, e.g. if I were picking someone up from the airport and the present time is more convenient for me to do so than when they actually did arrive or when they will arrive.
The second sentence would probably use the first definition of "could". (It technically could use the third in a manner similar to the above, but due to how the idiom is used, it is almost certainly the first definition. As JavaLatte stated above, it would be used when there is the possibility of someone arriving, but not certainty as to whether or not it has actually occurred.
