Question about sentence structure - [X曜日] + は,が,に.
I am a bit confused about expressions below. Are these all correct? Do these have a slightly different conotation? 私は金曜日に店に行く。 ； 金曜日は店に行く。 ； 金曜日が店に行く。 ； 私は金曜日が店に行く。 ； 私は金曜日は店に行く。
You can't use が with 金曜日 in this case. Since が is the subject particle, the sentence would mean "Friday goes to the store," which is very odd and (almost) definitely not what you're trying to say.
私は金曜日は店に行く would be incorrect because I don't think you can use は twice in a simple sentence (if it were a compound sentence, that would be fine). A simple sentece like that can't really have two topics
私は金曜日に店に行く is different from 金曜日は店に行く because the latter marks 金曜日 as the topic, so you would use the former when the topic is you and the latter when the topic is Friday. I'd be happy to elaborate further if you'd like.
Hope this helps!
I tried to create a good example but just ended up stuck (and admittedly a bit confused).
This is one of the best explanations of the function of "topic" in Japanese, and hopefully it'll answer your questions: http://nihonshock.com/2010/02/particles-the-difference-between-wa-and-ga/