Past tense forms of adjectives

I understand how to use the adjectives in the past tense (oishii to oishikatta) but I have problem when the adjective occurs with da/desu.

Can one say ''oishikatta desu''? Why is the adjective in the past tense when desu is in the present? Or am I supposed to say ''oishikatta deshita''? Or simply ''oishikatta'' without using any ''da'' form.

Many thanks to whoever is willing to help! =)

"Oishikatta desu" (it tasted good) is correct.

Please keep in mind that "i"-adjective is somewhat like a verb on its own ("oishii" means "to be tasty"); it has its negative form (oishikunai) as well as past tense (oishikatta) and negative past tense (oishikunakatta). The "desu" in "oishikatta desu" is added only for polite speech. If you are talking with a friend and want to say that something tasted good, you can simply say "Oishikatta!" and it is a complete sentence on its own :)

i-adjective:

(In informal speech)
Oishii (tastes good)
Oishikunai (does not taste good)
Oishikatta (tasted good)
Oishikunakatta (did not taste good)

(In polite speech)
Oishii desu
Oishikunai desu (or, oishiku arimasen)
Oishikatta desu
Oishikunakatta desu (or, oishiku arimasen deshita)

"na"-adjective changes form differently. na-adjective *needs* the be-verb (da, or desu) to complete the sentence.

(In informal speech)
Kirei da (is beautiful)
Kirei dewa nai (is not beautiful)
Kirei datta (was beautiful)
Kirei dewa nakatta (was not beautiful)

(In polite speech)
Kirei desu (is beautiful)
Kirei dewa arimasen (is not beautiful)
Kirei deshita (was beautiful)
Kirei dewa arimasen deshita (was not beautiful)