Active voice: I eat an apple.
Passive voice: An apple is eaten by me.
Test: If you are unsure and you can add “by zombies” it is the passive voice. It also reminds you that passive voice is a bit lifeless.
This recommendation is nothing new see Rule 14 in “Elements of Style” published in 1920. The Elements of Style was listed as one of the 100 most influential books by Time Magazine in its 2011 list.
Active Voice is also a good writing practice for journal publications. Have a look at the Nature research web page How to write your paper – Instructions for authors: “Nature journals prefer authors to write in the active voice (“we performed the experiment…”) as experience has shown that readers find concepts and results to be conveyed more clearly if written directly. “