a 所 explanation

[nipponman]

What, that which, all that, whatever (nearest thing to a Chin. relative pronoun, introducing a noun clause): 不知所當為 (當說) do not know what to do, what to say;

This is from linyutangs dictionary. But to me it is a little vague on proper usage, could someone explain this how to uses this word? Also is it an everyday word or formal speech or just useless? Any help would be appriciated,

Nipponman

[Claw]

The closest translation for 所Ｘ that I can think of is "that which is Ｘ."

So, 不知所當為/說 would mean "to not know that which is done/said" = "to not know what to do/say." This construction is somewhat formal though.

Incidentally, this is why the word 所有 means "all" or "everything" since it literally means "that which is had," and 所以 means "therefore" since it literally means "that by means of which is." Both of these words have now become rather idiomatic in Modern Chinese though, especially 所以, since it comes originally from a Classical Chinese grammar construction. Most people do not think of 所以 as meaning "that by means of which is," but rather as a single word meaning "therefore."

More often, you'll find 所 in front of verb-的-object constructions; for instance:

我所認識的漢字不多 = The Chinese characters that which I know is not much.

The 所 in these cases is usually optional. 我認識的漢字不多 is sufficient usually, and sounds less formal. However, if the object is implied, sometimes it sounds better to leave in the 所: 我所認識的不多.

[nipponman]

Thank you Claw! I always used to wonder about 所以.

[ever00t]

所以 means hence

[Claw]

...and "hence" is a synonym of "therefore"... what's your point?

[HashiriKata]

"hence" is a synonym of "therefore"... what's your point?

To give "therefore" a companion in case it feels lonely