My old japanese teacher (who was japanese) told us that が is much more specific than は
For instance
かおりさんが好きです = I like Kaori. (She's the one for me!)
q. かおりさんが好きですか =Do you like Kaori?
a. えぇ、かおりさんは好きですがはるかさんも好きです = Eh, Kaori's cool but I also like Haruka
公園はきたいです= the park is dirty.
公園がきたいです= the park is the dirty thing (say from a list of parts of a city, the others are clean.)
Unless you're trying to be a japanese wizard and use exciting language forms and features of japanese, stick with は as the subject marker and use が for potential form. eg "can you play the guitar?"
Which brings me to another interesting point. the difference between ことができる and ~える for potential. The same japanese teacher told us that while this isn't assesed in any way in the syllabus it will make you sound more fluent to native speakers later in life.
the ~える topic is used for transitive verbs only. ie. play the guitar, I can play the guitar. Not なおれる (can get better by itself, from なおる). This refers to things you can physically do (or someone else). Kinda like that thing in english "Could you pass the salt?" ; "I could, but I won't =P"
ことができる is more about being able to do things, not so much physically though. The only way to explain it is by example. テレビが見えません = I can't see the Tv. Physically there might be someone in the way, you might've gone blind. テレビを見ることができません = I can't watch tv. Maybe you're grounded from watching tv, maybe you're too busy at the moment, perhaps you'd wake somoene up if you did. You are physically able to, but you can't. They most always translate the same into english (and the context tells us which), but not in Japanese
Kinda confusing, but pretty interesting stuff
