A sentence like Sono Eikokujin wa wakaranakatta (That Englishman wa didn't understand) is better that Sono Eikokujin ga wakaranakatta.
compare Sachi wa nihon ye kaerimashita (Sachi wa to Japan returned) with Sachi ga nihon ye kaerimashita which in some cases (more cases than with `wa') suggests that Sachi is unique in her returning, or at least that Sachi makes up a complete list of (relevant) people who have returned.