What is the difference between 適切、適当 and 適宜?

Question

As the title says, I've been having some trouble with recognizing the difference between 適切、適当 and 適宜 as in dictionaries they are all translated to "appropriate, suitable".

According to this site

• 適当{てきとう} is equivalent to

  ほどよくあてはまる

• 適切{てきせつ} means something like

  過不足なく、きちっとあてはまるようす

• and 適宜{てきぎ} is

  「適当」や「適切」、「適度」という言葉すべての言葉の意を含めて「ちょうどよい」という意味を表す便利な言葉

but that only leaves me with more questions because the dictionary translates ほどよく as "rightly; properly; moderately​" and both きちっと and ちょうど as "just right".

According to this site the difference between 適切 and 適当 is that the level of "rightness": in case of 適切 is more precise. The site provides following examples of use:

「適当な広さの土地」
「適切な指導を行う」
「適宜に取り計らう」

but that still doesn't really help me, I'm pretty sure 「適切な広さ」 and 「適当な指導」 are correct as well and I don't really see what "more precise" could mean when comparing these.

I've looked on other sites as well, but the examples they provide are similarly vague. I would really appreciate it if someone could provide some clarification.

Answer 1

• 適当: This is actually a tricky word because it has two seemingly opposite meanings. 適当 especially in casual speech usually means "careless", "irresponsible", "lightheaded", "random", etc. 適当 in formal writings usually means "suitable", "corresponding", "fitting", etc. For example 適当な店 can mean "suitable restaurant" or "randomly-chosen restaurant" depending on the context, and it sometimes confuses even native speakers. See: Why do 適当 and いい加減 refer to both considerate and inconsiderate things?
• 適切: This is a safer choice that only means "appropriate". If you want to play safe in formal writings, you should be using this.
• 適宜: This has a different meaning from others. It's a formal word which means "according to the situation", "on a case-by-case basis", "taking a proper action for each step", etc. This is used when someone does not want to go into detail. Also note that this almost always works as a standalone adverb that does not take に, at least in modern Japanese. 適宜に is recognized by some dictionaries but it sounds weird to me. For example, 適切に敬語を使う simply means "to use keigo appropriately", whereas 適宜敬語を使う means something more nuanced like "to use keigo depending on the situation" or "to use proper keigo when(ever) necessary".