Archmage Callinon wrote:
English conjugates are a lot trickier, and there tend to be a lot more exceptions to what can only laughingly be called "rules" in English.
Japanese on the other hand has very few exceptions to rules and conjugates tend to be pretty straight-forward if you see enough of them. The grammar is also pretty easy to decipher. The hardest part for me was always the vocabulary (partially due to the aforementioned conjugation issues)
Regular English verbs have exactly four forms - basic form (bare stem), 3rd-person singular (stem+"s", the sole carryover from the Old English system of infections for person and number), gerund-participle (stem+"ing"), and past-participle (stem+"ed"), and there is only one class of regular verb. Some 75% of all English verbs are regular. The vast majority of irregular verbs still only have four or five forms and all but four of them are perfectly regular in the non-past (i.e. stem, stem+"s", and stem+"ing"); these verbs generally have a separate form for the simple past tense instead of using the past-participle, change the stem vowel instead of adding -"ed", and cluster into large families with specific patterns of vowel changes. Tense, aspect, and mood are expressed with compound forms combining a base verb with one or more modal verbs, while voice is expressed via sentence structure; valency is generally flexible.
The copula, "be", is by far the most irregular verb in the entire language; two other common verbs, "do" and "come" are somewhat irregular, but not highly so ("do": do,does,doing,done, [simple past] did; "come": come, comes, coming, come, [simple past] came). The next most irregular verbs after "be" are "do" and "go" (go, goes, going, gone, [simple past] went), both of which are regular in two or three forms.
Adjectives have at most three forms: basic form (bare stem), comparative (stem+"er"), and superlative (stem+"est"). Longer adjectives will only have the basic form. Adjectives do not decline like in most other Indo-European languages, so adjective-noun agreement is never an issue. Only a small handful of adjectives are irregular.
Nouns fall into two categories - mass nouns and count nouns. Mass nouns have two forms, basic form (bare stem) and genitive (stem+"'s"). Count nouns have four forms, but when regular, three of those forms are phonemically identical - singular (bare stem), plural, genitive singular, genitive plural (the last three are each written differently, but phonemically identical for regular count nouns: stem+"s"). The overwhelming majority of nouns are regular, and genitive forms are regular without exception. Nouns never change by case - semantic role is determined by word order. Determiners (a/an, the, my, etc.) are obligatory unless speaking generically.
Pronouns have only three forms - direct, oblique, and genitive. They are irregular, but they are a closed set, and there's only seven of them: 1st-person singular (I/me/mine), 1st-person plural (we/us/ours), 2nd-person (you/you/yours), 3rd-person masculine singular (he/him/his), 3rd-person feminine singular (she/her/hers), 3rd-person neuter singular (it/it/its), and 3rd-person unspecified (they/them/theirs).
The "ed" (past participle verb) and "s" (3rd-person singular verb; plural and genitive noun) have varying phonetic realizations depending upon the immediately preceding sound that are without exception regular, pronounced /iz/ or /id/ after a dental with the same manner of articulation (/t/ and /d/ for "ed", /s/ and /z/ for "s"), /s/ or /t/ after an unvoiced non-dental consonant, and /z/ or /d/ elsewhere. Pronunciations do not change when words are compounded.
The spelling system is actually far more regular and exception-free than most people give it credit for, especially given that we are forced to write a language that has in excess of 40 unique phonemes (with different dialects not even making the same phonemic distinctions) using only 26 characters, in particular 14 vowel classes with an alphabet that only contains 5 vowels and 2 consonantal vowels.
Regular Japanese verbs have five forms - imperfective, continuative, terminal/attributive, hypothetical, and imperative. While nearly all Japanese verbs are regular, they fall into three classes - ichidan, godan, and stative (aka "adjectives"). A verb's class is generally readily apparent, but certain godan verbs resemble (or are homophones of) ichidan verbs. Many verb endings also vary depending upon a verb's class. Tense, aspect, mood, and voice are all expressed with a combination of endings and modal verbs, which, unlike in English. Valency is fixed, though there are several pairs of verbs that only differ in valency.
The copula, which is filtered on this site, is by far the most irregular verb in the language; two other common verbs, "suru" (to do) and "kuru" (to come) are also highly irregular, having no regular forms whatsoever. ("suru": sa/shi/se, shi, suru, sure, shiro/seyo/sei; "kuru": ko, ki, kuru, kure, koi)
Adjectives fall into two categories - i-adjectives (native Japanese "adjectives") which are grammatically a type of verb (see above), and na-adjectives (mostly borrowed from Chinese) which are grammatically a type of noun.
Nouns do not inflect. Semantic role is determined by post-positional particles. Determiners do not exist; features such as number and definiteness are determined by context.
Pronouns are grammatically identical to nouns. Unlike in English, they are an open class, with several pronouns having the same meaning - the choice of pronouns encodes significant honorific information.
Phonetic change over time means that u-row godan verbs have the unexpected (though totally regular) imperfective form of stem+"wa" (because the stem historically ends in -"w" - however in modern Japanese /w/ is only pronounced before /a/), which itself unexpectedly becomes stem +"o" when adding the volitional ending "-u" (which is one of the endings that changes with verb class; it's '-yoo" for ichidan verbs); in fact, since the godan volitional ending always changes the final vowel of the continuative form from "a" to "o", most learning materials simply present the volitional ending as a sixth form, despite this process being entirely regular.
Similarly, the final mora of the continuative form of godan verbs changes in unexpected but mostly regular ways when adding an ending that starts with "t" - "i" "chi" "ri" become a "t" (i.e. geminate the following consonant), "bi" "mi" "ni" become "n" and cause the following "t" to become voiced, "ki" becomes "i", and "gi" becomes "i" and voices the following "t" - and again, despite this being regular, most learning materials present the -"te" and -"ta" endings as seventh and eight forms, and describe the provisional ending as "-ta-form + ra" rather than "continuative form + tara".
Pronunciations can change in ways sometimes regular and sometimes not when words are compounded.
Kana spelling is for all intents and purposes completely regular - Kanji, on the other hand are not, with a kanji's pronunciation varying wildly with usage.