[akstylish]

I've looked at some examples of katakana-English transitions, and it's rather confusing. For example, coffee in katakana is コーヒー. But if I didn't know it I would've written it as　コピ. Another is veil(ベール）. If I only saw katakana I would've translated it as bell. So what are the rules?

edit: oh I forgot f is pronounced as h in Japanese. Never mind the first example.