Passive voice
martin always wears casual clothes(my grammar book says passive voice is impossible here)
casual clothes are worn by Martin (wear is a transitive verb, so why is not possible?)

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I disagree with your grammar book!

"Casual clothes are worn by Martin" is perfectly correct grammatically. HOWEVER, it is true that most teachers would mark it wrong because STYLISTICALLY, most English speakers think active sentences sound better than passive sentences.

For example, this is an article where the author Steven King complains about the passive voice: http://www.westga.edu/~jsewell/King_Passive_Voi...

Most style guidebooks will suggest that you avoid the passive voice unless you don't know exactly who is doing the action.
- Martin wears Nike shoes. (good grammar, good style)
- Nike shoes are worn by Martin. (good grammar, bad style)
- People in China make Nike shoes. (good grammar, bad style - the word "people" is redundant)
- Nike shoes are made in China. (good grammar, good style)

Another situation where you might use the passive voice is if the object is much more important than the agent. Here are some (correct) sentences I found by Googling:
- Heavy military-style belts were worn by bureaucrats as well as soldiers. (from a Wikipedia article on Roman clothing)
- The shoes in this photo were worn by rapper Childish Gambino during a performance in Tallahassee. (from a blog entry describing the shoes)