Yeah, I definitely have that problem here sometimes. The teachers will ask me how we say something, and I'll say, "Like this~", and then they'll say, "But why not this~". And then I will just be like, "...because." Luckily, I've gotten a little better at picking out why stuff is better or worse, but it's still wicked frustrating when you don't know or there is no reason.

On a more specific note, I think that "such as" and "such a" are different because "such as" allows you to give a general example, and "such a" means you're referencing a specific thing in terms of what you were just talking about? Like, "There are many large cats in the world, such as tigers" versus, "In a large city, public transportation is essential. Such a city requires myriad forms of efficient and timely transportation." Probably you worked that out yourself, though. ^^;

Disco fever~ XD