良い has two readings, "ii" and "yoi" so "yokatta" is yo-(i)-katta. Also; it would be kakko ii and kakko yokatta, just like other i adjectives. and ii - yo (i) ku naru, yo (i) ku nakatta etc You probably know, "katta" makes an adjective past and used for adjectives ending in "i" only. zannen -> zannen datta. (=deshita) nigiyaka -> nigiyaka datta. (=deshita) omoshiroi -> omoshiro (i) katta. (=omoshirokatta desu) akai -> aka (i) katta. (=akakatta desu) and so on.

For your additional question: It's just simple past tense "It was good." It just means something was good at a certain point in the past. よかったですitself doesn't imply anything about current state. きのうは　てんきが　よかったです。そして、きょうも　いいです。 Yesterday, the weather was good. And it's good today, too. きのうは　てんきが　よかったです。でも、きょうは　よくないです。 Yesterday, the weather was good. But it's not good today. 「きのう　いった　レストラン、どうでしたか？」 -「よかったですよ。ぜひ　いってみてください。」 How was the restaurant you went yesterday?--It was good. You should try there. あのレストラン、まえは　よかったけど、オーナーが　かわってから　だめになったね。 That restaurant used to be good before, but it became bad after the owner was changed.

Time to get some grammar books or look at some grammar sites, they should explain stuff like this. ^^ As said before　良かった is just the past tense of 良い。 So yes, like this is good vs. that was good. The かった form is used for i adjectives (aka 形容詞　けいようし）when you put them in the past tense. (Well this form is used for the past tense at least most of the time) Na adjectives (called　形容動詞 けいようどうし but many Japanese don't know this word） have a slightly different past tense, here you put the past form of だ/です on the end. To speak Japanese you need to learn how to quickly change the forms of i adjectives, na adjectives and verbs. Verbs are the most complicated because they have quite a few different endings so you will have to spend more time doing those.

良い, 良いです. past tense is 良いでした. the stem す changed to し + た. In modern Japanese, the shorten form for です is た. 良かった , it is the shorten form for 良いです, meaning: it is good. Additon: 良かったです, in your question 良かった is a noun, and です is a verb. 良 and かった have to be together in forming a noun. Just to find the meaning of かった, it doesn't make sense.

I can't type in Japanese right now but the form you are talking about is the past tense. so ii turns in to yokatta. and tanoshii turns into tanoshikatta.