>alt.usage.english

Thanks tsuwm. I think this does relate to my usage of the word. I'm not so bothered about informal writing but I do like to know why the wretched machine is trying to alter my careful prose.

I ran your post through the grammar checker and it did not try to replace the "which" in "The family, which is the basic unit of human society, is weakening." I suspect that all it does is look to see if the clause is set off with commas, if not, it recommends that the user considers replacing "which" with "that".