When 大 is read as たい versus だい？


There is a general rule that for a kanji compound of the form 大＊, you tend to
read 大 using 音読み (タイ・ダイ) if ＊ is 漢語 and with 訓読み (おお) if ＊ is 和語. See e.g.
[this question on goo](https://oshiete.goo.ne.jp/qa/3353923.html) (in
Japanese). (Of course there are many exceptions, and this rule is not special
to 大---see [General guidelines for choosing 訓読み vs. 音読み in kanji
reading](https://japanese.stackexchange.com/q/43197/9199))

Are there any rules of thumb or broad etymological reasons for which 音読み to
use for 大 at the beginning of a kanji compound?

For instance, you typically read the following examples as タイ＊:

> 大木、大金、大量、大気、大変

and in these as ダイ＊

> 大事、大体、大地、大名、大学

One subjective impression I have is that if you have some 漢語, at least in the
case of 二字熟語, and modify it with 大 at the beginning that it is typically read
as ダイ, e.g., 大人気 or 大家族. (Compare this with 大使館, say, where the root word
seems to be 大使.)




The 音読み used for specific words varies based on when the word was borrowed
from Chinese. ダイ is the [呉音](https://en.wikipedia.org/wiki/Go-on) and タイ is
the [漢音](https://en.wikipedia.org/wiki/Kan-on). There's no hard rule for how
to tell which pronunciation to use.




The 音読み used for specific words varies based on when the word was borrowed
from Chinese. ダイ is the [呉音](https://en.wikipedia.org/wiki/Go-on) and タイ is
the [漢音](https://en.wikipedia.org/wiki/Kan-on). There's no hard rule for how
to tell which pronunciation to use.

