1. In an hour, we arrived at the base.
2. After an hour, we arrived at the base.
3. In an hour, we will arrive at the base.
4. After an hour, we will arrive at the base.
(Which preposition do we have to use? Which ones are correct?)
They are all correct. However, 1 is unclear, depending upon the preceding sentence.
2 and 3
In the examples you provided, both "in" and "after" can be used interchangeably to convey the same meaning. Both prepositions are correct in this context.
1. "In an hour, we arrived at the base." - This sentence suggests that the arrival at the base happened within the specific time frame of one hour.
2. "After an hour, we arrived at the base." - This sentence indicates that the arrival at the base occurred following the passage of one hour.
So, both sentences convey the idea that one hour passed and then the arrival at the base took place.
As for the future tense:
3. "In an hour, we will arrive at the base." - This sentence implies that one hour from now, the arrival at the base will occur.
4. "After an hour, we will arrive at the base." - This sentence conveys the idea that one hour will pass, and following that, the arrival at the base will happen.
Again, both sentences are correct and convey a similar meaning, with "in" emphasizing the time frame and "after" emphasizing the sequence of events. Ultimately, the choice between "in" and "after" depends on the preferred style or context of the sentence.