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(The following is from "The Quick and Dirty Guide to Japanese Grammar" by Tad Perry.)

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-U Verbs (GROUP I)

    ( -RU verbs (GROUP II) are by far the easiest to conjugate: you drop -ru and add something else. Simple. )

    -U verbs (GROUP I) aren't that easy, but almost. Typically, you drop -u and add something else. The problem is that there might be a phonetic change (such as when ha becomes pa, or ta becomes da) for some of the types. See the other chart for simple English meanings of the conjunctions; this one just tries to keep the conjunction rules clear.

    First, there are some -u verbs that end in -ru. We have to get that out of the way right now. Real -ru verbs (where -ru gets dropped) always end in "iru" or "eru".  If you see one that ends in: "aru" (exist), "uru" (sell),  or  "oru" (break), it's guaranteed not to be a -ru verb. It's probably a -u verb (with the exception of super-polite oddballs like gozaru/gozaimasu.)

    Verbs like "iru" (exist or need, depending on context) can be either type.
In fact, "need" is an -u verb (gerund is itte) and and "exist" is a -ru verb (gerund is ite). If you hear one of these, (like "hairu") be sure to ask what the gerund ("haitte") or past tense ("haitte") is too, so you can remember what type it is.

    So far I've only met three verbs that ended "-eru" that were -u verbs instead of -ru verbs. These are:
    keru/kette (kick),
    heru/hette (decrease)
    heru/hette (elapse)

    Except for these "iru/eru" verbs, every other type of verb is obvious by hearing only one form.

    Including -u verbs which might happen to end in -ru, these are all the types there are:
-su 
-ku (these two types conjugate similarly)
-gu
-bu (these two types conjugate similarly)
-mu 
-nu 
-ru (these three types conjugate similarly)
-tu
-vowel+u

-su is the only one that is well-behaved.

-(s)u will change simply to -s(a), -s(i), -s(e), -s(o) in all cases with no odd behavior to remember.
 
 
 

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Here are the replacements:
 

    replace -u with -i to add "-masu", "-yasui", etc. (works straight across, no phonetic changes in any of the types).
Dictionary form -masu form Eigo
     hanasu hanashimasu [he] talks
        kiku kikimasu [he] walks
        oyogu oyogimasu [he] swims 
        yobu yobimasu [he] calls 
        nomu nomimasu [he] drinks 
        sinu  sinimasu [he] dies 
        tsukuru tsukurimasu [he] makes
        matu matimasu [he] waits
        harau haraimasu [he] pays

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Replace -u with -ite for the -TE (imperative) form.
(Types not following the rule, but acting similarly, are grouped.)
Verb -te form Eigo comment
hanasu hanasite  talk! normal
kiku kiite walk! (replace entire -ku)
oyogu oyoide  swim!  (replace entire -gu, be sure to use -ide)
yobu yonde call!  (replace entire -bu, be sure to use -nde)
nomu nonde  drink!  (replace entire -mu, be sure to use -nde) 
sinu sinde die!  (replace entire -nu, be sure to use -nde) 
tukuru tukutte  make!  (replace entire -ru, be sure to use -tte)
matu matte wait!  (replace entire -tu, be sure to use -tte)
harau haratte pay!  (replace -u, be sure to use -tte)

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Replace -u with -ita for past tense.
Types not strictly following the rule, but acting similarly, are grouped.
hanasu  hanasita talk 
kiku kiita walk  (replace entire -ku)
oyogu oyoida  swim  (replace entire -gu, be sure to use -ida)
yobu yonda call  (replace entire -bu, be sure to use -nda)
nomu nonda drink  (replace entire -mu, be sure to use -nda)
sinu sinda  die  (replace entire -nu, be sure to use -nda)
tukuru  tukutta make  (replace entire -ru, be sure to use -tta)
matu  matta wait  (replace entire -tu, be sure to use -tta)
harau haratta pay  (replace -u, be sure to use -tta)
(In fact, I query-searched and replaced "e" with "a" on the gerunds to get this if that helps any. The rule breakers are breaking the rule uniformly now.)

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Replace -u with -itara for "if". (Or, just add "ra" to the past.)
(Types not strictly following the rule, but acting similarly, are grouped.)
hanasu  hanasitara  talk 
kiku  kiitara  walk  (replace entire -ku)
oyogu  oyoidara  swim  (replace entire -gu, be sure to use -idara)
yobu  yondara  call  (replace entire -bu, be sure to use -ndara)
nomu  nondara  drink  (replace entire -mu, be sure to use -ndara)
sinu  sindara  die  (replace entire -nu, be sure to use -ndara)
tukuru  tukuttara  make  (replace entire -ru, be sure to use -ttara)
matu  mattara  wait  (replace entire -tu, be sure to use -ttara)
harau  harattara  pay  (replace -u, be sure to use -ttara)

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Replace -u with -itari for "do things like X"
(Or just add "ri" to past).
hanasu  hanasitari  talk 
kiku  kiitari  walk  (replace entire -ku)
oyogu  oyoidari  swim  (replace entire -gu, be sure to use -idari)
yobu  yondari  call  (replace entire -bu, be sure to use -ndari)
nomu  nondari  drink  (replace entire -mu, be sure to use -ndari)
sinu  sindari  die  (replace entire -nu, be sure to use -ndari)
tukuru  tukuttari  make  (replace entire -ru, be sure to use -ttari)
matu  mattari  wait  (replace entire -tu, be sure to use -ttari)
harau  harattari  pay  (replace -u, be sure to use -ttari)

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Replace -u with -eba for "if". 
(works straight across, no phonetic changes in any of the types)
hanasu  hanaseba  talk 
kiku  kikeba  walk
oyogu  oyogeba  swim 
yobu  yobeba  call 
nomu  nomeba  drink 
sinu  sineba  die 
tukuru  tukureba  make
matu  mateba  wait 
harau  haraeba  pay 

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Replace -u with -oo for "let's do X".
(works straight across, no phonetic changes in any of the types)
hanasu  hanasoo  talk 
kiku  kikoo  walk 
oyogu  oyogoo  swim 
yobu  yoboo  call 
nomu  nomoo  drink 
sinu  sinoo  die 
tukuru  tukuroo  make 
matu  matoo  wait 
harau  haraoo  pay 

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Replace -u with -e for rude orders "do X dammit".
(works straight across, no phonetic changes in any of the types)
hanasu  hanase  talk 
kiku  kike  walk 
oyogu  oyoge  swim 
yobu  yobe  call 
nomu  nome  drink 
sinu  sine  die 
tukuru  tukure  make 
matu  mate  wait 
harau  harae  pay 

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Replace -u with -anai for negative.
(This is now an adjective.)
(works straight across, only one phonetic change in one type)
hanasu  hanasanai  talk 
kiku  kikanai  walk
oyogu  oyoganai  swim 
yobu  yobanai  call 
nomu  nomanai  drink 
sinu  sinanai  die 
tukuru  tukuranai  make 
matu  matanai  wait 
hara harawanai  pay  (stick a wa in there, not just an "a". Makes it easier to say, too. Lucky us!)

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Replace -u with -eru for "can do X".
This is now a -ru verb.
(works straight across, no phonetic changes in any of the types)
hanasu  hanaseru  talk 
kiku  kikeru  walk
oyogu  oyogeru  swim 
yobu  yoberu  call 
nomu  nomeru  drink 
sinu sineru  die 
tukuru  tukureru  make 
matu  materu  wait 
harau  haraeru  pay 

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Replace -u with -aseru for "make (someone) do X".
This is now a -ru verb.
(works straight across, only one phonetic change in one of the types)
hanasu  hanasaseru  talk 
kiku  kikaseru  walk
oyogu  oyogaseru  swim 
yobu  yobaseru  call 
nomu  nomaseru  drink 
sinu  sinaseru  die 
tukuru  tukuraseru  make
matu  mataseru  wait 
harau harawaseru pay  (Don't forget to throw a wa in there!)

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Replace -u with -areru for "X is done (often to someone)".
This is now a -ru verb.
(works straight across, only one phonetic change in one of the types)
hanasu  hanasareru  talk
kiku  kikareru  walk 
oyogu  oyogareru  swim 
yobu  yobareru  call
nomu  nomareru  drink 
sinu  sinareru  die 
tukuru  tukurareru  make 
matu  matareru  wait 
harau harawareru  pay  (Don't forget to throw a wa in there!)

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Replace -u with -aserareru for "be made to do X by someone".
Same result as just doing the two conjunctions separately but is taught in most books as a specific conjunction.
This is now a -ru verb.
(works straight across, only one phonetic change in one of the types)
hanasu  hanasaserareru  talk 
kiku  kikaserareru  walk 
oyogu  oyogaserareru  swim 
yobu  yobaserareru  call 
nomu  nomaserareru  drink 
sinu  sinaserareru  die 
tukuru  tukuraserareru  make 
matu  mataserareru  wait
harau harawaserareru pay  (Don't forget to throw a wa in there!)

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Now that was long, but it was mostly cut-and-paste because even these are well-behaved when you organize them correctly. When you boil down what needs to be done to these verbs to conjugate them correctly, the information required is really minimal--even for the oddballs. That's all you need to be able to conjugate almost every verb in Japanese.

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-RU VERBS

IRREGULAR VERBS

ADJECTIVES

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