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.
> The wrong bit is that "tanoshikunai desu" should really be "tanoshiku arimasen".
While it should technically be -ku arimasen, it's actually rarely used, and -kunai desu is more "mainstream".
> 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"
Trivia: all these forms are really variations of "suki de aru". "datta" comes from "de atta", "de ha nai" is really "de nai" with a "ha" for emphasis. "de ha arimasen" is really "de aru", with the "ha" for emphasis, and "aru" conjugated with the "masu" auxiliary at the negative form.
> 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.
Technically speaking, the -i and the de are not combining at all. The -u form (ウ音便) comes from the -ku form (連用形), where the k is removed. Then the preceding sound also changes (like in arigataku -> arigatou ; oishiku -> oishuu, etc.). The typical forms used in keigo are -u gozaimasu and -u zonjimasu (where there is no "de" to combine in the latter ;) )
I think the Japanese learn the ウ音便 in 国語 or 古文, so I don't think it's some obscure trivia that few people know. In fact, you can hear it in e.g. 時代劇 dramas.
Also, the form is pretty common in Kansai dialect (without gozaimasu). In fact, wikipedia claims[1] it comes from there and the gozaimasu was added in Kantou.
> The main problem beginning Japanese learners often face is that they are taught polite form before plain form.
I often heard that the rationale for doing so is that learners don't sound impolite. It kind of makes sense, but on the other hand, I think it's also part of the "日本語が上手ですね" problem, that is, for the Japanese, the polite form is rather advanced.
Anyways, I do agree that for learners it would all make more sense to start from the basics and learn the polite forms later. Textbooks tend not to, though, sadly.
If I could upvote something multiple times, it'd be this post. The conjugations are well documented and can be intuited with enough exposure, but the insight as to why (い-verbs) and common usages (お早うございます) is something clearly lacking from the literature.
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.