~gatai ～がたい, ～難い

In Japanese, ~gatai ～がたい, also spelled ~gatai ～難い, is an auxiliary adjective that means something "hard to do" because you aren't willing to do it, or it's "hard to happen" because you don't believe it normally happen.

(not to be confused with gattai 合体, which means "to combine.")

Manga: Maid in Abyss, メイドインアビス (Chaper 15, 不動卿)

Usage

As an auxiliary adjective, ~gatai ～がたい is attached to the masu stem of verbs, also called their "connective form," ren'youkei 連用形.

suru
する
To do.
shimasu
します
To do. (polite form.)
shi
し
Doing.
shi-gatai
しがたい
Hard to do.
shi-gatai
し難い
(same meaning.)

Conjugation

The word ~gatai ～がたい can be conjugated just liked any i-adjective.

~gatai
～がたい
Hard [to do].
~gatakatta
～がたかった
Was hard [to do].
~gatakunai
～がたくない
Not hard [to do].
~gatakunakatta
～がたくなかった
Was not hard [to do].

Although it's grammatically valid to inflect it to negative (is not, was not), in practice those inflections are rarely used, as ~gatai is mostly used to say something is difficult to do, not to say something isn't difficult to do.

～難い

When ~gatai is written with kanji it becomes ~gatai ～難い, written with the kanji for the word "difficult," muzukashii 難しい.

Confusingly, ~nikui ～にくい, another auxiliary adjective that also means something is "hard to do," can also be written with kanji as ~nikui ～難い. That is, ~gatai and ~nikui are written the same way when they're written with kanji, they're homographs.

Katai 難い

The word katai 難い, an adjective meaning "difficult," is where the auxiliary adjective ~gatai ～難い comes from.

The ka か becomes ga が due to a change in pronunciation called rendaku 連濁, which adds a "diacritic," dakuten 濁点, to the suffix.

To make matters more confusing, katai 難い, "difficult," is homonym with katai 固い, "solid." And to make them even more confusing, in English there's word that's synonymous with both "difficult" and "solid," the word "hard."

～がたい vs. ～にくい

The word ~gatai is very similar to ~nikui ～にくい, which also means something is hard to do, but there are some differences.

Note that, regardless of the differences, most of the time people will say ~nikui rather than ~gatai anyway because ~gatai is more used in writing, in some set phrases, and by characters in manga who speak in a more solemn, literary way, while ~nikui is more used in normal speech.

Resisting to Do It

One difference between ~gatai and ~nikui ～にくい is that ~nikui is something that's difficult to achieve or to happen due its nature, while ~gatai expresses that something is hard to do because you find resistance doing it.

For example:

iu
言う
To say.
ii-nikui
言いにくい
Hard to say, because of the nature of what's being said.
Anyone would find this hard to say.
ii-gatai
言いがたい
Hard to say, because I'd rather not say it.
Maybe I'm not so sure about it, so I'd like to refrain from making a comment.

This means that ~nikui is more objective, difficult for anyone, while ~gatai is subjective, difficult for you.

Furthermore, ~nikui means that, while complicated to do, it's possible to achieve, while ~gatai means that, because of reasons, you think it's probably not gonna happen.

I'd Rather Not

You'll see that ~gatai is often used with verbs where the difficulty isn't that the process is complicated or that the execution in practice is troublesome (~nikui), the difficulty is merely your willingness to do it. (~gatai).

shinjiru
信じる
To believe.
shinji-gatai
信じがたい
Hard to believe.
Unbelievable.
I'd rather not believe it.

yurusu
許す
To forgive.
yurushi-gatai
許しがたい
Hard to forgive.
Unforgivable.
I'd rather not forgive it.
yurushi-gatai koui
許しがたい行為
An act [that's] hard to forgive.
Unforgivable act.

mitomeru
認める
To acknowledge.
mitome-gatai
認めがたい
Hard to acknowledge.
I'd rather not acknowledge it.
I'll never admit it!

Psychological Verbs

It's also used with verbs related to the mind, in which the difficulty is entirely psychological.

wasureru
忘れる
To forget.
wasure-gatai
忘れがたい
Hard to forget.
Unforgettable.
I'd rather not forget it.

kangaeru
考える
To think.
kangae-gatai
考えがたい
Hard to think.
Unthinkable.
I'd rather not think it's so.

rikai suru
理解する
To comprehend.
rikai shi-gatai
理解しがたい
Hard to comprehend.
Incomprehensible.
To me, it makes no sense. I can't comprehend it.

taeru
耐える
To endure. To tolerate. To bear.
tae-gatai
耐えがたい
Hard to endure.
Intolerable. Unbearable.
ENOUGH IS ENOUGH!!!
tae-gatai kutsujoku
耐えがたい屈辱
An humiliation [that's] hard to endure.
An unbearable humiliation.

Manga: Maid in Abyss, メイドインアビス (Chaper 15, 不動卿)

do shi-gatai
度し難い
Hard to persuade-and-make-understand.
Even if you tried to persuade them, you don't think they'd understand.
They're irredeemable. Beyond salvation.
- do suru
  度する
  To persuade someone to understand.
  To make them see the truth, the logic, to make them see the light.
  To redeem. To save. (from saido 済度, "salvation," Buddhist term.)

Note that, although ~gatai is generally translated as if something "can't" happen, it doesn't literally mean it "can't" happen, just that you believe it's so difficult to happen it probably won't happen.

That is, it's different from a negative potential conjugation of a verb:

shinjirarenai
信じられない
Can't believe [it].

Although, in practice, the example above has pretty much the same meaning as shinjigatai.

Probably Not Gonna Happen

Another case is that ~gatai implies something is difficult to occur, that it's rare for it to occur.

aru
有る
To have.
ari-gatai
有り難い
Hard to have. Hard to come by.
- ari-gatou gozaimasu
  有難う御座います
  "Thanks" comes from the adjective above.

eru
得る
To acquire.
e-gatai
得がたい
Hard to acquire.

Note that, while ~nikui can also mean something is hard to happen, its meaning is slightly different. For example:

koware-nikui
壊れにくい
Hard to break. Rarely breaks.
Something that's hard to break because of how it's built, but, like all things, will eventually break, just not as easily as other, more breakable things.
koware-gatai
壊れがたい
Hard to break. Probably won't break.
Something you believe to be hard to break because you think it's absolutely unbreakable, or maybe you believe it would take a lot to break it, anyway, you don't think it's going to break easily, anytime soon or ever.

～がたい vs. ～づらい

The difference between ~gatai and ~dzurai ～づらい is that, while both are subjective ways to say something is difficult, ~dzurai means something is hard to do because it causes distress, whereas ~gatai means something is hard to do because you don't want to do it.

For example:

hanashi-nikui
話しにくい
Hard to talk.
Because I don't have the time to talk. Anyone in my position would have difficulty talking.
hanashi-dzurai
話しづらい
Hard to talk.
Because it's a subject that makes me uncomfortable.
hanashi-gatai
話しがたい
Hard to talk.
Because it's a subject I'd rather not talk about.
Because it's someone that I believe I probably won't be able to talk with.