Adjective Clauses With Prepositions

Prepositions are normally used after the relative pronouns (using them before relative pronouns is rather formal).

The man to whom I spoke is my uncle (RU-5) (formal).

The man who / that/- I spoke to is my uncle (less formal).

New York is a city to which I have traveled many times (formal).

New York is a city which I have traveled to many times (less formal).

The bus for which we were waiting was late (formal).

The bus that/which/- we were waiting for was late (less formal).

Sources: 

1. University of California, Irvine (2018). Adjectives and Adjective Clauses.Available at: https://www.coursera.org/learn/adjective-clauses/lecture/NZqfW/adjective-clauses-with-prepositions [Accessed 2 Oct. 2018].
2. Bbc.co.uk. (2018). Learning English | BBC World Service. [online] Available at: http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv286.shtml [Accessed 1 Oct. 2018].