Why 入った and not 入た past tense


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?




### 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).




### 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).

