This sentence "Last night at 9 PM, I ate dinner" means that the action began at 9 or finished at 9?
Does it mean that I started eating at 9?
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The use of the preposition at specifies a certain point in time. Since a meal cannot be eaten in/over/during a point of time, the most common way to interpret your sentence
Last night at 9 PM, I ate dinner
is that you are telling at what point in time that dinner started.
I cannot stress the following enough:
It would be rare to non-existent for a native speaker to utter your sentence and refer to 9 PM as the stopping point.
We often say sentences like yours (using at a specific time) to refer to the starting time of a meal or other event. This is probably so that other people know what time to arrive at the meal or event.
This includes both the past and non-past. Examples with at in addition to your sentence include
1 Lunch was at 3 yesterday.
2 We had breakfast at 6.
3 They would always eat at noon.
All the above refer to the start time.
4 Dinner is at 7 tonight.
5 I always eat lunch at 1 pm.
6 At midnight I like to have snack.
Again, the above refer to a point in time, and that point in time is the start time.
To indicate the stopping point with at, you would usually need to make it clear:
7 Last night I finished my dinner at 9pm.
8 Lunch is over at 12:30. Then you return to class.
Changing the prepositional phrase to around 9pm changes the meaning of the sentence to 9pm plus or minus about 15 minutes. But you did not ask about around 9pm.
As a side issue, it is not necessary to say PM when you specify last night since PM includes the hour of nine at night.
As a native speaker, I'd argue that the statement is generic enough to say that the answer is "all of the above".
What this is realistically saying is "Last night, around 9 PM, I ate dinner." - meaning that they could have:
There's no real way to know which is "correct" and if the person phrased it this way, it probably doesn't really matter. All they're saying is that, some time during the hour from 9-10 pm, they ate dinner.
If someone wanted to be explicit they would say something more specific like:
I worked so late last night that I didn't sit down to dinner until 9 pm.
There was so much food at the feast that I didn't finish eating dinner until 9 pm!
In these examples, you can clearly see that the action is beginning or ending at 9 pm, respectively.
Last night at 9PM, I ate dinner.
Last night at 9PM, I finished eating dinner.
Last night at 9PM, I had eaten dinner.
End or later.
Last night at 9PM, I was eating dinner.
Some time during the dinner.
Last night at 9PM, I had been eating dinner.
Some time during the dinner when an interruption occurs.
I think it's safe to say that telicity is involved here.
[On the other hand I don't think the use of light verbs (e.g., have for eat) matters in this case.]
Compare the following sentences:
*I conquered Elbonia for three days.
I conquered Elbonia in three days.
Eating or to eat is not a necessarily complete act.
Eating a meal or to eat a meal is necessarily a complete act. Otherwise you would only be eating part of the meal.
Notice that we would have to switch to a continuous or progressive description (or change the tense entirely) in order to be able to indicate anything other than the time at which the meal occurred.
I was eating around nine o'clock.
However we are considering the entire, complete act. In some sense it isn't relevant when it begins and ends--precisely because it is complete and self-contained. Thus it is unnecessary to force the beginning alone to be aligned with the time mentioned.
I ate at nine.
I did eat at nine.
These statements include all the information we need to understand the time of occurrence because they force us to consider the meal as an event taking place at a specific time.
Interesting sentence. It's the answer that I would give if I was asked "what did you do yesterday at precisely 9 PM?". In that case the answer would mean "At precisely 9 PM, I was eating. So most likely I started before 9 PM and finished after 9 PM, unless I took my very first or very last bite at exactly 9 PM".
On the other hand, if I said "at 8 PM, I came home from work. At 8:30 I had a shower, and at 9 PM I had dinner" - then it would mean that the shower started at 8:30, and the dinner started at 9 PM. It all depends on the context.