(at + time/in/on/at)
Video about how to pronounce the word 'at'
Video lesson about prepositions of time
http://www.engvid.com/prepositions-how-to-use-in-at-and-on-in-english/
Video lesson: on time or in time? confusing prepositions
http://www.engvid.com/prepositions-in-on-time
Grammar notes
https://www.englishclub.com/grammar/prepositions-at-in-on-time.htm
Fill in the blank exercise
http://www.perfect-english-grammar.com/time-prepositions-exercise-2.html