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I've learned that ru verbs should drop る and replace it with た.

But 入る becomes 入った and I'm not sure why.

The same goes for

  * 食べる → 食べた
  * しゃべる → しゃべた

Are these irregulars or am I misunderstanding something?
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2018-08-21T19:26:27.720
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2018-08-22T06:29:38.330
2018-08-21T21:25:19.410
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question
verbs conjugations tense past
Why 入った and not 入た past tense
The rule for '-ru' (ichidan) verbs and '-u' (godan) verbs is 'anything ending
in '-iru' or '-eru' _can_ be a '-ru' verb. But 入る and しゃべる are not; they are
'-u' verbs that conjugate as you would expect.
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2018-08-21T21:03:45.553
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2018-08-21T21:03:45.553
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answer
The name " _ru_ verb" is misleading.

This does **not** mean that every verb ending in _-ru_ conjugates the same
way.

Certain verbs end in _-ru_ , and that final _-ru_ disappears in entirety when
conjugating. These are called **一段活用** ( _ichidan katsuyō_ , literally
"monograde conjugation" or "one-step conjugation") verbs in Japanese grammars,
because there is only one vowel on the end of the verb stem (either _-i_ or
_-e_ ). These are often labeled as " **type 2** " verbs in English texts about
Japanese grammar.

One everyday example is 食べる ( _taberu_ , "to eat"), where that final _-ru_
just disappears entirely in conjugated forms:

  * _tabe **ru**_ "[I / he / she / they] eat", plain informal
  * _tabenai_ "[I / he / she / they] don't / doesn't eat", plain negative
  * _tabemasu_ "[I / he / she / they] eat", polite
  * _tabesaseru_ "[I / he / she / they] make someone eat", plain causative

... etc. As you can see here, the _-ru_ vanishes entirely in the conjugated
forms.

Other verbs end in _-ru_ or _-ku_ or _-su_ , etc., and that final piece
doesn't quite vanish entirely -- the consonant remains, but the vowel changes.
These are called **五段活用** ( _godan katsuyō_ , literally "quintigrade
conjugation" or "five-step conjugation") verbs in Japanese grammars, because
there are five different possible vowels on the end of the verb stem ( _-a_ ,
_-i_ , _-u_ , _-e_ , _-o_ ). These are often labeled as " **type 1** " verbs
in English texts about Japanese grammar.

One everyday example with _-ru_ is 入る ( _hairu_ , "to enter, to go into"),
where that final _-ru_ doesn't disappear entirely, and only the vowel changes:

  * _hai **ru**_ "[I / he / she / they] enter", plain informal
  * _hai **r** anai_ "[I / he / she / they] don't / doesn't enter", plain negative
  * _hai **r** imasu_ "[I / he / she / they] enter", polite
  * _hai **r** aseru_ "[I / he / she / they] make someone enter", plain causative

As you can see here, the _-r-_ sticks around, and the vowel changes.

Things ultimately get a bit more complicated than this (such as the
conjunctive forms and plain past forms, where you often get geminate [i.e.
doubled] consonants instead of the final consonant, as in _haitta_ ), but
that's the basic framework. There's good information on Wikipedia, at
<https://en.wikipedia.org/wiki/Japanese_verb_conjugation>.
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2018-08-21T21:20:48.390
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answer
### tl;dr

入る _hairu_ is a consonant-stem verb, i.e. _hair·u_.

### Long version

Besides a handful of exceptions, there are two type of verbs

  * vowel-stem verbs ( _-e·ru_ , _-i·ru_ )
  * consonant-stem verbs ( _-k·u_ , _-g·u_ , _-s·u_ , _-t·u_ , _-n·u_ , _-(w)·u_ , _-m·u_ , _-r·u_ )

where I have used an interpunct · to separate the stem from the ending (and
the hyphen - means there is more coming before it).

These types follow **different inflection patterns**.

Of course there are no pure consonants in _kana_ , so written in _kana_ you
cannot show where the border between stem and ending is — for example a verb
ending in ～く may inflect to ～か (as in 聞 **く** → 聞 **か** ない), but written as
above, the stem ending in _-k_ does not change ( _kik·u_ → _kik·anai_ ). (This
is why it is called the stem. And the stem of vowel-stem/consonant-stem verbs
end in a vowel/consonant, respectively.)

Now, seeing a verb written in _kana_ , there is no way of telling whether a
verb ending in _-eru_ / _-iru_ is a vowel-stem verb or a consonant stem verb,
i.e. whether it is _-e·ru_ / _-i·ru_ or _-er·u_ / _-ir·u_.

The following vowel-stem–consonant-stem verb pairs show that this difference
simply has to be learned:

  * 着る _ki·ru_ ↔ 切る _kir·u_
  * 変える _kae·ru_ ↔ 帰る _kaer·u_

In particular, their past tenses are different

  * 着た _kita_ ↔ 切った _kitta_
  * 変えた _kaeta_ ↔ 帰った _kaetta_

Now, 入る is _hair·u_ (i.e. a consonant-stem verb), so its past tense follows
the same pattern as 切る, giving 入った.

However, the way the verb is written in _kanji_ gives some way of guessing
whether it is vowel-stem or consonant-stem. As a rule of thumb (for verbs of
three or more morae), if the _-i_ or _-e_ is part of the _kanji_ , then a verb
is more likely to be consonant-stem.

In particular, since はいる is a three mora verb, written 入る rather than *入いる, it
is more likely to be consonant-stem. The rule also correctly identifies 食べる as
a vowel-stem verb (→ 食べた) and 喋る【しゃべる】 as a consonant-stem verb (→ 喋 **っ** た).
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2018-08-21T21:23:21.383
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2018-08-22T06:29:38.330
2020-06-17T08:18:27.500
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