In short

These patterns all sit on the ます-stem and say how open or closed a possibility is. 〜かねない (-kanenai) warns that something bad could well happen; 〜得る／〜得ない (-uru / -enai) states cold logical possibility or impossibility; 〜ざるを得ない (-zaru o enai) says circumstances leave you no choice; 〜っこない (-kkonai) is a casual “no way”; and 〜ようがない (-yō ga nai) says the means to do it simply do not exist. Learn 〜かねる (-kaneru, a polite “I am unable to”) at the same time, because it looks like the opposite of かねない and is not.

The rule

Step 1 - Everything here builds on the ます-stem. Take the polite form, drop ます, attach. This is the same base that carried 〜すぎる and 〜やすい in lesson 40 and the compound verbs in lesson 48, so the machinery is familiar: なります -> なり, 分かります -> 分かり, 連絡します -> 連絡し. Only ざるを得ない and the “no choice but” neighbours of Step 8 (しかない, よりほかない, わけにはいかない) break the pattern, and Steps 5 and 8 say how.

Step 2 - 〜かねない: a risk, and always a bad one.

ます-stem + かねない, polite 〜かねません. It means “there is a real danger that…, it could well…”. The defining restriction is evaluative: the outcome must be one the speaker regards as undesirable. 大きな事故になりかねません (ōkina jiko ni narikanemasen, it could well cause a serious accident) is natural; ×成功しかねません is not, because succeeding is good - that is plain 〜かもしれません from lesson 32.

かねない is a warning word. It belongs to news writing, safety notices and careful speech, and it usually follows a description of the careless behaviour that creates the risk.

Step 3 - 〜かねる: the false friend you must learn together with it.

ます-stem + かねる, polite 〜かねます, means “cannot bring myself to, am not in a position to”. It is not about ability - it is a soft refusal, the standard business-Japanese way of saying no: お答えしかねます (o-kotae shikanemasu, I am unable to answer), 分かりかねます, いたしかねます.

Now look at the two polite endings side by side, because one syllable flips the meaning:

• 〜かねます = I cannot (refusal).
• 〜かねません = it may well happen (warning).

Morphologically かねない is the negative of かねる; semantically it is nothing like it. Saying ×お答えしかねません to a customer promises that you might blurt out the answer.

Step 4 - 〜得る／〜得ない: logical possibility, in writing.

ます-stem + 得る = “it is possible for this to occur”; ます-stem + 得ない = “it cannot occur”. This is usually not personal ability (lesson 37) - it is about whether the world admits the event at all: 誰にでも起こり得る問題 (dare ni demo okoriuru mondai, a problem that can happen to anyone).

The reading is the trap. In the plain non-past the kanji 得 may be read うる uru or える eru - あり得る is both ariuru and arieru, and both are correct. Everywhere else in the modern language - negative, past, polite, て-form - the reading is fixed at える: あり得ない arienai, あり得ました ariemashita. There is no ×ariuranai. Register is formal and bookish, except for あり得ない, which has escaped into everyday speech as an exclamation (“unbelievable!”).

Step 5 - 〜ざるを得ない: compelled, and not happy about it.

Take the ない-form, drop ない, add ざるを得ない (-zaru o enai): 行かない -> 行かざるを得ない ikazaru o enai; 食べない -> 食べざるを得ない tabezaru o enai; 来ない -> 来ざるを得ない kozaru o enai. The one irregular is する, whose base here is せ: せざるを得ない sezaru o enai - exactly the same せ that gave せずに in lesson 54.

It is a double negative (“cannot not do it”), so it means I have no choice but to, with an audible note of reluctance. Compare with lesson 35: 〜なければなりません reports a rule or a duty neutrally, while 〜ざるを得ません says external circumstances forced your hand. Register is formal written and formal spoken; it would sound stiff among friends.

Step 6 - 〜っこない: the casual flat denial.

ます-stem + っこない = “there’s no way”. Emphatic, dismissive, and plain style only: 分かりっこない wakarikkonai, できっこない dekikkonai, 勝てっこない katekkonai (built on the potential stem 勝て). It attaches freely to potential forms, which is where much of its force comes from.

Keep it out of writing and out of anything work-related; the neutral equivalents are 〜わけがない and 〜はずがない from lesson 53.

Step 7 - 〜ようがない: no means exist.

ます-stem + ようがない (also 〜ようもない), where よう is the noun 様 “way, means”. It says the action is blocked because there is no method available, not because you lack the skill:

• 連絡しようがありません。 (Renraku shiyō ga arimasen.) There is no way to contact him - I have no number, no address.
• 直しようがありません。 (Naoshiyō ga arimasen.) There is no way to fix it - the cause is unknown.

Contrast it with the potential negative. 連絡できません points at you: you are unable. 連絡しようがありません points at the world: the means are missing. English “I can’t” hides the difference; Japanese makes you choose.

Step 8 - The “no choice but” neighbours. V-plain + しかない (shika nai) is the everyday one: あきらめるしかありません, I have no choice but to give up. V-plain + よりほか(は)ない (yori hoka (wa) nai) is its formal written twin. And V-plain + わけにはいかない (lesson 53) is the social version - you are able, but you must not. The emotional patterns 〜ずにはいられない／〜ないではいられない (“can’t help doing”) are a different family and belong to lesson 65.

Forms & patterns

Building each pattern:

得る - うる only in the plain non-past, える everywhere else:

ざるを得ない across the verb classes:

What each one actually claims:

Register ladder - the same idea at five heights:

Examples

Common mistakes

• 大きな事故になるかねません。 (Ōkina jiko ni naru kanemasen.) -> 大きな事故になりかねません。 (Ōkina jiko ni narikanemasen.) - かねない attaches to the ます-stem, never to the plain form. Say the polite form in your head, drop ます, and attach: なります -> なり.
• この薬はよく効きかねません。 (Kono kusuri wa yoku kikikanemasen.) -> この薬はよく効くかもしれません。 (Kono kusuri wa yoku kiku kamoshiremasen.) - かねない is reserved for undesirable outcomes. A good possibility is かもしれません.
• その質問にはお答えしかねません。 (Sono shitsumon ni wa o-kotae shikanemasen.) -> その質問にはお答えしかねます。 (Sono shitsumon ni wa o-kotae shikanemasu.) - the refusal is かねます. かねません is the warning form and here promises the opposite of what you mean.
• 試合を中止しざるを得ませんでした。 (Shiai o chūshi shizaru o emasen deshita.) -> 試合を中止せざるを得ませんでした。 (Shiai o chūshi sezaru o emasen deshita.) - before ざる, する uses the base せ, never し. Same irregularity as せずに.
• 連絡するようがありません。 (Renraku suru yō ga arimasen.) -> 連絡しようがありません。 (Renraku shiyō ga arimasen.) - ようがない also takes the ます-stem: 連絡します -> 連絡し + ようがない.

Tips

• One syllable, opposite meanings: かねます vs かねません. Remember them by who is speaking. かねます is the clerk at the counter refusing you politely. かねません is the safety notice warning you. If a human subject is declining, it is かねます; if a situation is threatening, it is かねません.
• 得る has one flexible reading and four fixed ones. Only the plain non-past can be uru; the moment you inflect it - negative, past, polite, て-form - it is eru. In practice you will read あり得る and 考え得る as ariuru and kangaeuru, but say あり得ない as arienai.
• できない blames you, しようがない blames the world. Ability lives in the potential form (lesson 37); the absence of any method lives in ようがない. Note also the frozen phrase しようがない／しょうがない (shiyō ga nai / shō ga nai), which no longer means “there is no way to do it” but “it can’t be helped” - one of the most common expressions in spoken Japanese.
• Pick your height before you pick your pattern. The same thought - “impossible” - is っこない to a friend, わけがありません in conversation, and あり得ません in a report. Dropping っこない into a business email is the register error learners actually make here, and it reads as flippant.