Conjugating adjectives

How do I conjugate Japanese -ii and -na adjectives to plain form? Do I just change the -desu after to -da? eg: Would omoshiroi desu become omoshiroi da?

No, 面白いだ is ungrammatical because you have a predicate followed by the copula. The です in 面白いです is not the copula but simply a politeness marker. The "plain form" of 面白いです is just 面白い.

By the way, although "i-adjectives" are predicates, "na-adjectives" are not, so your guess is right for those: for example 綺麗です does become 綺麗だ.

Thanks Firas. For a beginner like me it can be really confusing to know which bits to change in order to change the form! What is the plain form for 'ano machi wa kirei ja arimasen deshita'? Would it be 'ano machi wa kirei kunai datta' ?

"ano machi wa kirei ja arimasen deshita" is weird, you don't contract "de wa" to "ja" in polite speech...

For conjugation purposes, "na-adjectives" work basically like nouns. The negative of "Xだ" is "Xじゃない" and the past of that is "Xじゃなかった".

Thanks Firas. I asked my Japanese friend and they said it should be きれいではない. I am so confused, I thought ではない -dewa nai was only used for the present tense but the sentence I need to change was in polite past form. Also why is -ja in this sentence? This is from a textbook by the way. In the early questions that I had to change to plain form, eg: おもしろい です, my Japanese friend told me the plain form is just おもしろい but I thought it should be おもしろい だ. Do you know if there's a website that lists how to change adjectives like the verb conjugator websites?

Indeed ではない is the present (or the "non-past", as it is often called since it also serves as the future tense).

じゃ is a contraction of では, in the same way that in English "isn't" is a contraction of "is not". There are very many such contractions in Japanese, but they are not used in polite speech. In any case, the meaning is not changed.

Any decent textbook should list the conjugation rules for adjectives, but if yours doesn't, Google gave me this : http://rcl.pliable.us/J-adj.html

You have the adjective types mixed up there :)
"omoshiroi" is an i-adjective. "kirei(na)" is a na-adjective.

So:
(Polite speech) i-adjective
Omoshiroi desu. (It is interesting.)
Omoshirokunai desu. (It is not interesting.)
Omoshirokatta desu. (It was interesting.)
Omoshirokunakatta desu. (It was not interesting.)

(Plain speech)
Omoshiroi. (It is interesting.)
Omoshirokunai.(It is not interesting.)
Omoshirokatta. (It was interesting.)
Omoshirokunakatta. (It was not interesting.)

With i-adjective, the word itself conjugates, and the only difference between polite speech and plain speech is whether you add desu or not. (So the "desu" here is just for politeness.)


(Polite speech) na-adjective
Kirei desu. (It is pretty.)
Kirei dewanai desu. or Kirei janai desu. (It is not pretty.)
Kirei deshita. (It was pretty.)
Kirei dewanakatta desu. or Kirei janakatta desu. (It was not pretty.)

(Plain speech)
Kirei da. (It is pretty.)
Kirei dewanai. Or Kirei janai. (It is not pretty.)
Kirei datta. (It was pretty.)
Kirei dewa nakatta. or Kirei ja nakatta. (It was not pretty.)

With na-adjective, you need to have "da" (plain speech) or "desu" (polite speech) as the "to be" verb." So it's the verb that conjugates :)

Thanks AK, your explanation was really clear. Obviously I still have a lot to learn, haha!