There are many prepositions in English and using them correctly is problematic. 

They can completely alter the meaning of the sentence.

As I set a goal not to make mistakes in writing English sentences where English learners often do, I have to practice using them and basically I think I should study them in chunks.

In English, to talk about something happening at a certain time we can use at, in and on, and I know the grammatical rules, but I can't use them correctly on the spot.

Learning prepositions in sentences and as a part of phrases and fixed expressions is the best way to avoid slip ups.