This is awesome! I've always wanted to try this. The only real complaint I have is that "da" is not actually a copula in the strictest sense. It's a contraction of "de aru". Similarly "na" is not a modifier. It's a contraction of "ni aru". "aru" is the verb which is the closest you get to a copula in Japanese - it means "it exists" for non-animate noun-phrases.

So if you say "sakana da", it does mean "It is fish", but so does just "sakana". The copula is implied. The "da" is completely optional and is actually only added for emphasis -- the literal translation is kind of like "That it is fish exists". In literary Japanese you would say "sakana de aru", "de" being the particle that links a verb the the means with which the verb is executed. For example "basu de iku" means "will go by bus" -- bus is the means by which we will go. In "sakana de aru" or "sakana da" we are basically saying that "fish" is the means of its existence.

The "na" modifier is also interesting. It is really "ni aru" where "ni" is essentially the "direction" in which something exists. "Something like a fish" would be "sakana no you". If you want to say "It is a fragrance something like a fish" you could say "sakana no you na kaori". Although I'm not aware of any modern Japanese that would express it like this, this is equivalent to "sakana no you ni aru kaori" -- "It is a fragrance that exists in the direction like a fish". Hopefully you can understand.

The interesting part of this is that adding "ni aru" to the end of a noun phrase just turns it into a verb phrase. And the even more interesting bit is that the only thing that can modify a noun phrase is a verb phrase.

But, you may have heard of "i-adjectives" -- these are adjectives that end in i. In actuality, these are not adjectives! They are verb phrases! So the word "cute" is "kawaii". However, the actual word is "kawai" and the inflection is "i". That's why when you want to say "not cute" it becomes "kawaiku nai" -- the "i" turns into "ku" because you are inflecting a verb.

This in turn is why you modify nouns directly with "i-adjectives". "kawai sakana", or "cute fish". Other adjectives are actually noun phrases in Japanese. "yumei na sakana" or "famous fish". This is, again, exactly the same as "yumei ni aru sakana" -- "The fish exists in the direction of fame".

So the rules are even simpler than presented in this blog post.

By the way, for anyone trying to learn Japanese and who wants to go beyond phrase-book level: learn plain form first and polite form later (if ever). Japanese makes absolutely no sense if you learn polite form first. It's incredibly logical (even the polite form extensions) if you start with plain form.