A number of prepositions may be used to denote time: from Monday; after my return; during the night; till tomorrow; before the bell rings; a quarter to ten. In most cases the preposition required is easily found out. The following prepositions, however, needs special attention:
1.At: ‘At' is used with a definite point of time; for example, with hours, moments. ‘At' is also used with the names of the festivals.
2.On: ‘On' is used with more general points of time than ‘At'; for example, with days and dates.
3.In: ‘In' is used with parts of the day, months, years, seasons. ‘In' is also used with the future tense to show the period in which an action will happen.
Note: ‘In' and ‘Within': ‘In' means ‘at the end of'; ‘within' means ‘before the end of'.
4.By: ‘By' refers to a point of future time and denotes the latest time at which an action will be over.
5.For: ‘For' is used with periods of time to show the duration of an action. It is mostly used with perfect continuous tenses thought it may be found with other tenses as well.
6.Since: ‘Since' marks the point of time at which an action began. It is used only if the action has continued till the time of speaking, hence it is found with perfect continuous tenses. Unlike ‘for' it can never be omitted.
(a)At 7 p.m.; at this moment.
(b)At Diwali; at Durga Puja.
(a)On Monday; on 1st may; on the annual day; on a May afternoon.
(a)In a week; in four hours.
(b)I shall be back in a week. (when a week is over)
(c)I shall be back within a week. (before a week is over)
(a)The competition will be over by 6 p.m. (It should be over before it is 6, but the latest time at which it can be over is 6).
(b)They will have declared the result by tomorrow evening.
(a)The child has been sleeping for three hours.
(b)I have been busy the whole morning. (also, for the whole morning)
(a)She has been teaching in the college since 1965.