"Pendant que" or "parce que" or "alors que" are subordinating conjunctions, acting as links between the main clause (il écrit) and the subordinate clause (je lis).
These links are mainly causal (because) or temporal (while).
Note that there are some in two words (examples above) or in one word: "lorsque, quand, puisque..."
Why "whereas" is wrong in this sentence ? In a previous exercise, we used "whereas" to say "pendant que...". I no longer understand ! (I specify, I'm French)...