Is there a general rule for deriving xasu→xaseru intransitives such as 死なせる from 死なす?


There are a number of verbs where there is a 〜xasu → xaseru transformation to
produce an transitive verb from an intransitive, eg:

  * 死なす→死なせる
  * 生かす→生かせる
  * 飲ます→飲ませる

Is this some kind of generalized rule? (perhaps a classical conjugation?) Or
is it something that must be handled on a case-by-case basis?




There is another base form before ~asu, and yes, there is rules for that.

[code]

    - 死ぬ (die)　　  ⇒　死なす　(let ~ die)  
                     ⇒　死なせる (make ~ die)
    - 生きる (live)   ⇒　生かす　(let ~ live)   
                     ⇒　生かせる (make ~ live)
    - 飲みます(drink) ⇒　飲ます　(let ~ drink) 
                     ⇒　飲ませる　(make ~ drink)
    
[/code]




It seems that there are several confusions in the question.

First, 死なす and 死なせる are almost interchangeable (to make/let … die), and so are
飲ます and 飲ませる (to make/let … drink). (I wrote “almost” just in case; honestly
speaking, I cannot think of any situation where one is correct is the other is
not.)

Second, the relation between 生かす (to make/let … live; to utilize) and 生かせる
(can make … live; can utilize) is very different from the other two pairs of
words. The pair 生かす/生きさせる has the same relation as the two pairs 死なす/死なせる and
飲ます/飲ませる.

Both 飲ます and 飲ませる are made from the verb 飲む. 飲ます is a verb derived from the
verb 飲む (to drink). The derived verbs like 飲ます where the derivation adds the
meaning “make/let” are called 使役動詞 in the Japanese grammar. 飲ませる consists of
飲ま (the 未然形 of 飲む) and an auxiliary verb せる (make/let). The same applies to
死なす and 死なせる, both of which are made from the verb 死ぬ (to die).

Almost the same explanation applies to 生かす and 生きさせる, both of which are made
from the verb 生きる (to live), but there are two complications here.

  * The verb 生かす has an additional meaning: to utilize. Therefore, 生かす and 生きさせる are not always interchangeable.
  * The phrase 生きさせる consists of 生き and させる. 生き is the 未然形 of 生きる, and させる is an auxiliary verb which has the same meaning as せる. The distinction between せる and させる depends on the verb to which it is attached, but I will not go into details now.

生かせる is a verb derived from the verb 生かす using another method of derivation,
and it means “can make … live” or “can utilize / can be utilized.”

_Edit_ : In revision 1, I used the term “conjugation” incorrectly.

