The main problem beginning Japanese learners often face is that they are taught polite form before plain form. Polite form is a natural extension of plain form, but if you start with that, it's actually quite mind bending to back track to plain form. The secret is to abandon polite form entirely until you are relatively fluent with plain form and then add polite form back in.

For example, "tanoshii" is present/future tense. "tanoshikatta" is past tense. If you want to make it polite, then you just add "desu". Super easy.

While it is grammatically incorrect, it is completely acceptable in normal conversation to do the same with the negation. "tanoshikunai" is the negation. Past tense negation is "tanoshikunakatta" (ye gods, I can't read romaji...). You can do exactly the same thing to make it polite -- just jam "desu" on the end. That's what every child will do. The wrong bit is that "tanoshikunai desu" should really be "tanoshiku arimasen".

For "na" adjectives, it works differently. "suki" is present tense. To make it polite: "suki desu". Past tense is "suki datta". To make it polite "suki deshita". Negation is "suki de wa nai" (seriously, romaji makes me cringe...). Polite negation is "suki de wa arimasen" (though you can very much get away with the mistake of saying "suki de wa nai desu" -- again, every single child speaks this way).

Past tense negation is "suki de wa nakatta". Polite is "suki de wa arimasen deshita" (but again, the easy way is "suki de wa nakatta desu").

So, why is it like this? The reason is that "i" adjectives were originally verbs that had a different set of inflections/conjugations. Very obscure piece of trivia (that most Japanese people don't even know) is that "ohayou gozaimasu" is actually one of those conjugations -- it's actually "(honourific) o hayai de gozaru" in polite form. The "i" ending mixes with "de" to produce the "ou" ending. Anyway, the point is that you have to inflect it because it is literally a verb that is modifying a noun.

"na" adjectives on the other hand are actually adjectives. They are called "na" adjectives because you have to add "na" when modifying the noun. For example, "suki na hito". The "na" is actually a contraction of "ni aru" -- because in Japanese you can only modify nouns with verb phrases.

So this is why there is a difference between the negation of "i" adjectives and "na" adjectives. "ku" is the verb combining form of the old style "i" verbs (like "te" is on modern verbs). So "tanoshikunai" is really "tanoshiku nai" -- you are combining the "tanoshi" verb with the "nai" verb. On the other hand "suki" is actually an adjective, not a verb, so you have to say "suki de wa nai" -- you can't combine them.

Past tense is exactly the same. In "tanoshikunakatta", it's really combining 2 verbs and conjugating the last one (as per the rules" -- "tanoshiku nakatta"). If you want to make it polite, the polite past tense of "nai" is "arimasen deshita" (but you can get away with "nakatta desu" in virtually every situation).

With "na" adjectives -- "suki de wa nakatta", we've conjugated the only verb. Again to make it polite you can say "suki de wa arimasen deshita" (or "suki de wa nakatta desu" if you want to sound like an uneducated bumpkin like me).

Hope this helps! Avoid polite form until you can handle plain form and it's almost all completely logical ;-)

Edit: Fix past tense in the examples of incorrect, but acceptable polite forms.