Hi everyone!

I wrote this sentence 'она часто думает а нём'. My friend said preposition a doesn´t exist and I should write 'о нем'. I asked her by email and I think she´s a little busy and will take a certain time to answer me, so I´d like to know why my sentence is wrong.

Thanks in advance!

* I made a mistake when I considered to write but instead of about. Now I got it! Thanks to all of you who helped me!