What is the meaning and grammar of “them as can’t” in the following text:
"Them as can, do; them as can't, teach." Alas, more teachers fall into this category than ever before
It is a common proverb:
Prov. People who are able to do something well can do that thing for a living, while people who are not able to do anything that well make a living by teaching. (Used to disparage teachers. From George Bernard Shaw's Man and Superman.)
- Bob: I'm so discouraged. My writing teacher told me my novel is hopeless. Jane: Don't listen to her, Bob. Remember: those who can, do; those who can't, teach.
(McGraw-Hill Dictionary of American Idioms and Phrasal Verbs)
For the use of them see Using “them” instead of “those”.
It is northern England dialect, most likely Yorkshire.
Them as has/he as can/we as know = Those that have/he that can/We that know.
As @user240918 points out, it is an idiomatic saying, a kind of folk wisdom, especially in northern England.