Causative conjugations in Japanese are complicated. Historically, many transitive verbs are derived by adding す to a stem, creating naturally causative-like expressions. So, 燃やす describes the event of someone putting something on fire. The thing doesn't just spontaneously ignite. If it were to, you would use the intransitive 燃える. Yet, the causative forms for both 燃える and 燃やす involve using a form ultimately coming from the same source as す that makes so many transitive verbs.
|Verb Root||Intransitive||Intransitive Causative||Transitive||Transitive Causative|
to make X burn
to burn X
to make someone burn X
One would hope that this could be straightforward for every verb, but reality tells otherwise. There is also the short causative 燃えさす for 燃えさせる, but this form is usually △・X for most speakers. If the verb in the base form were a 五段 verb not ending in す, then it would be OK to anyone. Due to the fact that せる comes from す, the す in more and more transitive verbs is starting to become せる. This lesson will explain the ins and outs of this problem.
Intransitive → Transitive and or Intransitive Causative
Historically, the causative endings were さす and す. The す ending transitives and causative phrases is the same word, ignoring conjugation differences. Odd vowel changes may happen when attaching す to make transitives, but they all semantically resemble causative phrases.
|To burn → To burn it||燃える → 燃やす||To boil → To boil it||沸く → 沸かす|
|To get off → To drop off||降りる → 降ろす||To melt → To melt it||溶く → 溶かす|
On the left side, we have verbs with transitive forms and intransitive causative forms. That means 燃えさせる・燃やす and 降りさせる・降ろす are all words with significant difference in meaning. On the right side, we have verbs with no intransitive causative form. These forms, though, look and act like intransitive causative phrases. Verbs like on the left are able to have both forms with the aid of さ insertion. What then would need to happen in form for verbs on the right to have a causative form?
If ～せる comes from す, then ～せる should be able to be used just like ～させる to create intransitive causative phrases. Consider 澄む (to be transparent). 澄む's transitive form is 澄ます, which is causative in nature. A causative form of 澄むwould be identical in meaning to a transitive form. So, 澄ませる would be the potential form of 澄ます. Nevertheless, because 澄ます is causative in nature, the す is seen by more speakers as the short causative and changing it to 澄ませる for the causative. Even so, no one would ever have a problem telling when 澄ませる is meant to be the causative or potential.
So, which transitive verbs ending in す are vulnerable to changing to end in せる? Certainly, any intransitive verb with no distinct transitive and intransitive causative form is vulnerable. As we look at individual cases of this change, the majority fall under this situation. Even so, -asu is becoming more stigmatic of dialectical speech. Thus, verbs with intransitive causative and transitive verbs are now starting to have the transitive form ending in せる. 沸かす and 溶かす are more vulnerable to change than 燃やす, but the verb 甘やかす (to spoil), which you'll see later as well, has already changed for some speakers to 甘やかせる. This is despite the fact that 甘えさせる exists. This means that one day speakers may use 燃やせる to mean "to burn X".
Comparing Old and Modern Conjugations
To tell whether an intransitive verb has both a causative and transitive form, let's compare the modern and old conjugations for the causative. させる and せる come from さす and す. They did not conjugate exactly like す（る） as in the present or the past, but they were still similar due to them sharing the same origin. The chart below show shows the old causative conjugations and the modern ones (short and long). As you can see, although the short causative is a retention of the old, some change has occurred.
The modern さす is almost nonexistent in Standard Japanese. Its usage is usually dialectical. As for the modern す, it is still really productive. Its 連体形 and 已然形 are not as common, but they're usually not felt as being wrong. The 命令形 is questionable to some speakers, so it will be left in parentheses. Aside from the short causative-passive usage of the 未然形 with the modern す, the modern long causative forms are the most used (and times the only ones you can/should use).
As you can see, さす has almost died out. Any instance of it is dialectical. The modern す is also starting to die. Signs of this are evident in the chart. One base is already questionable, and as stated above, aside from the short-causative passive, you're likely to use the modern せる. If the language is hating on -asu, then it's no wonder transitive verbs ending in this are starting to end in -aseru.
So far, we have seen what verbs are being most affected, and we've even looked at the causative conjugations of the past and present and found yet another source of the problem. Now, we are going to look at individual examples to see how well they fit into the framework described thus far. There is a lot of individual variation, so it is almost impossible to fully express all the details of the options.
When 下一段 is Most Common
Transitive verbs which have shifted from す → せる are most certainly the trigger for all the change we see in other verbs.
When Both 下一段 & 五段 are Most Common
These next verbs are undergoing change to only ending in せる, but the traditional form is still holding on strong. Unlike the previous verbs, form preference is more hectic.
|Intransitive||Intransitive Causative||Transitive||Transitive Causative||New Transitive|
Knowing what form to use is not an easy task as a learner. In the end, the best thing that you should do is listening to what Japanese speakers are using. If various forms exist, they're likely going to be different. For instance, 甘えさせる and 甘やかす exist, but they're vastly different. The first is a good thing and the second is a bad thing. Intransitive verbs with causative-like transitive forms may sometimes, but the causative should still come from the transitive version of the verb. These include verbs like 鳴る and 散る, but they're in the minority. For instance, 照る means "to shine" and 照らす means "to brighten", but the causative is 照らさせる. You cannot find 照らせる to mean 照らす. It is the potential as expected.
Over time, more verbs will switch over to having their transitive forms end in せる. This lesson was about transitive verbs having corrupted forms due to the causative. This confusion arose due to common etymology, and there is various degrees of acceptance of variant forms. This is what you can expect in a wider change affecting the entire language. Sadly, this will only be truly easy centuries later when this change has fully taken effect.
Forms are listed from most to least common.
To finish supper.
2. 知識を｛ひけらかす 〇・ひけらかせる △｝。
To show off one's knowledge.
3. 子供を｛甘やかす 〇・甘やかせる △｝。
To spoil one's/a children.
It doesn't bore the crowd.