Explaining those are a bit difficult, so I'll just point you to http://www.guidetojapanese.org/particles.html , there's a good explanation there. Actually I think you can use を in that case, it wouldn't be incorrect, but using が seems more common. You can't use を with the potential form of する (できる) though, so personally I just use が for every potential verb I use. :P (That is, every verb that doesn't already have a special particle, like へ/に行く.) So... 日本語を/が話せます。 日本語ができます。 大学へ/に行けます。 Hope that was clear enough.