A parallel yet separate phenomenon to /ra/ deletion, there are three unique grammatical circumstances in which the syllable /sa/ is inserted into a phrase due to semantic proximity with similarly constructed structures.
The phenomenon being described can easily be noticed by a well-versed learner or speaker of Japanese. Take for instance the following example.
(We) will be closed today.
The verb 休む is a five-grade conjugating verb, and when combined with ～（さ）せていただく in humble speech to indicate one's own action in the most respectful way possible, it should connect together with the ～せる iteration of the causative form to its 未然形 base, following its verb class. Thus, in Standard Japanese, we get:
(We) will be closed today.
Putting aside how many speakers would find 休業いたします to be most proper for this exact scenario, why is it that ～させていただく is being used essentially as a set phrase regardless of the class of the verb being used?
Before this question is answered, this is still only one of three scenarios in which 'unnecessary' /sa/-insertion is occurring. This lesson aims to introduce you to those scenarios as a real-world Japanese grammar issue that, at this point, may very well be affecting your speech as an advanced learner and how to address it properly.
As alluded to in the introduction, ～（さ）せていただく exists as a quintessential phrase in modern humble speech. Although some speakers may shrug at using it in every instance one wishes to mention personal action, its place as the most honorific sentence ending in Modern Japanese is certain.
For the most part, a native speaker will not stumble over conjugation rules, especially when the conjugating itself is not too difficult.
食べる ⇒ 食べさせていただく
書く ⇒ 書かせていただく
見る ⇒ 見させていただく ？
読む ⇒ 読ませていただく
From this chart, we see that one-grade verbs take ～させる and five-grade verbs take ～せる. You may also notice that する and 来る are not mentioned. This is because する would manifest as させていただく anyway. Then, as for 来る, although 来させていただく would be valid morphologically, it would be unnatural for the same reason 見させていただく is marked with a question mark because the base verb already has a separate humble speech equivalent.
Surprisingly, there are estimates which state that as much as 30% of the population are using ～させていただく even if the verb is one-grade. This phenomenon is an example of analogical leveling. In other words, rather than having two forms of what is deemed to be the same phrase, simplifying it to just one form makes things simpler.
The result of this leveling has the following effect on five-grade verbs.
'Correct Form' for 五段活用動詞
/sa/-Insertion with 五段活用動詞
歌う ⇒ 歌わせていただく
歌う ⇒ 歌わさせていただく
話す ⇒ 話させていただく
話す ⇒ 話ささせていただく X
In your intermediate studies, you would've learned about the auxiliary verb ～そうだ. This auxiliary serves two functions, neither of which share the same conjugation rules with what they follow. In the first sense, it means "to seem," and in the second sense, it marks hearsay.
This yogurt seems like it's healthy.
(I) hear that this yogurt is healthy.
When you go somewhere which doesn't seem it would have toilet paper, it's best to bring some with you.
(I) hear that there is no toilet paper wherever you go.
For the meaning of "to seem," we know that ～そうだ attaches to the stem of adjectives. However, in the case of 良い (to be good) and 無い (to not be there/to not have), さ is inserted as a cushion syllable. The reason for why this arose with just these two adjectives is still uncertain.
Some speakers take issue with the analysis that this rule only affects these two adjectives. Instead, they feel that it affects all two-syllable adjectives. To their defense, there are only two other such adjectives in the entire language: 濃い and 憂い. So, are there examples of 濃さそう and 憂さそう?
8a. この紅茶は濃そうですね。 ◎
This black tea seems thick, doesn't it?
When Google searching 濃さそうですね, less than 3K results show as opposed to the 7K+ results for 濃そうですね. When you remove the sentence endings, the results are 28K+ and 200K+ respectively. These results suggest that speakers overwhelmingly choose 濃そう as the preferred form. When observing results for 濃さそう more closely, many come about from discussions regarding its grammaticality. However, such discussions cannot be use to discredit the phrase's existence as many posts can be found where speakers claim it's the preferred form in conversation where they live. In standard education, 濃そう is, in fact, deemed the correct form of the expression, which may have an effective on how prevalent it is used in the written language, which Internet posts would be a subset of.
When researching 憂い, it must be noted that the adjective is rarely used in isolation. If used at all, it is usually paired with the prefix もの- to get 物憂い meaning "melancholy." Since its use is so rare in the spoken language, one may presume that the form 憂さそう might still exist if there are indeed speakers that believe inserting さ onto such two-syllable adjectives is obligatory. Upon Google searching it, only three examples appear, all used with 物憂い instead. Each example is found in novel-like prose, which may be indicative of novice mistakes from a younger generation. Although there is far from enough examples to suggest that this form could be deemed 'correct' anytime soon, results of sentences written by native speakers indicates that this grammatical scenario for /sa/-insertion is playing out in a certain percentage of the population.
Although it's established that なさそうだ is proper Japanese when it is the result of ～そうだ attaching to the standalone adjective 無い. It is also worth noting that 'proper' /sa/-insertion is observed with 無い when paired with the supplementary verb ～すぎる to indicate "too..."
When your attire is neat, you shouldn't look as if you have no money.
I have so little money, I can't stand it!
However, it has become commonplace for /sa/-insertion occur when ない is being used as an auxiliary verb, and it is occurring with both ～そうだ and ～すぎる.
11a~b: It doesn't seem like it will rain.
11c: It seems like it won't rain (at all).
11d: It doesn't look like it will rain.
Though 11a. would be the 'proper way' to express the meaning of "doesn't seem like it will rain," it is overshadowed by every other option. Furthermore, despite 11b. displaying 'improper' /sa/-insertion, it is the predominant form for the majority of speakers in West Japan as well as the totality of Japanese speakers. What's more, it must be understood that this is not just in reference to the verb 降る. Rather, ～なさそうだ is become standard in general.
That employee is doing his job so unenthused.
Even though it doesn't seem first outwardly, when you ride it, it's surprisingly sporty and it's the kind of car even a street racer would be ok with in spite of themselves.
Has KPOP spread that much overseas that even the most unfamous looking idol group will be viewed tens of millions of times?
So, how about ～なさすぎる's acceptance rate? The prescriptive grammar approach claims that all instances are ungrammatical. However, some believe it's far less unnatural than using ～なそうだ when used with one-syllable verbs such as する, 来る, and 見る. It would appear that even for middle-aged speakers, not even these verbs can be used naturally with ～なさすぎる, but as for casual, spoken language among young people and certain areas of Japan, ～なさすぎる is very pervasive, particularly in hyperbole.
15a. 彼は何も出来なすぎる！ 〇
He's unable to do way too much!
That guy knows way too little of the world.
For the next few examples, only ～なさすぎる will be used. All examples are indicative of casual speech.
I watch so little TV that I don't even know what tomorrow's weather will be like.
Ryota, I'm so worried because I'm getting so few notifications from you!
Japanese people just study way too little...
This damn gacha, there's too few two-stars coming from it!
The person in charge of "IMABI," he does so few updates, all you can do is worry that he's even alright.
There are also three set phrases which utilize ～ない in a auxiliary~supplementary-like fashion: つまらない (boring), もったいない (too good), and かたじけない (indebted). Because their morphological relation with ～ない is complicated, some believe that ～なそうだ・なすぎる is proper as you cannot separate ～ない from them. Others feel, though, that ～なさそう sounds more natural for the same reason that the phrase sounds better in general.
Kenta was staring at the waste of beer he had spilled.
It's the kind of movie that's so bad and boring that you want to watch it.
She seemed so bored that she was staring outside all throughout work.
The man lowered his head in gratitude.
※It must be noted that 忝（かたじけな）い is seldom used, meaning you won't see much examples either way with ～な（さ）そうだ or ～な（さ）すぎる.
Then, there are regular adjectives that happen to end in /nai/ such as 少ない (few), 危ない (dangerous), and 汚い (dirty), which lack the auxiliary ～ない altogether. As for these words, the phenomenon of /sa/-insertion is not seen nearly as frequently, and doing so is deemed to be ungrammatical by the majority of speakers. As such, any examples of this are deemed as mistakes caused by the outward resemblance.
The weather sure looks dangerous.
Nonetheless, there are some mistakes that can be made all the time. Perhaps 26 will be completely natural to most speakers 100 years from now.