jobs they have been working at

No, they have different meanings normally.



No, you can be said to work at a job, meaning to work.
There are many related examples I could give, but I'm not sure of the value of that. It's a well-discussed topic, the world of work.

Thank you, but what's the difference between on/at:

-He's working at losing weight.
-Learning to play the piano isn't easy. You have to work at it.
-You need to work on your pronunciation a bit more.
-‘Have you sorted out a babysitter yet?’ ‘No, but I'm working on it.’

I think you have answered your own question. Each phrase/ idiom has to be individually learned.
But, if you inadvertently switch an at or an on, it doesn't really matter as far as the meaning is concerned.

Um...what does

-if you inadvertently switch an at or an on, it doesn't really matter as far as the meaning is concerned

mean? SWITCH ON here does not have its ordinary meaning regarding light switches.

Switch means change, exchange.
"Light switch" is a particular use, not the fundamental one.

And note, SWITCH ON does not appear in my post.

Um...

Quote:I think you have answered your own question.

No, I have not. The OALD9th gives meanings that are almost the same; but you said they are different; but how different? I see absolutely no difference from those example sentences.

I have answered your questions up to now in a spirit of helpfulness, but I don't like your attitude nor your approach.
So I will not answer any further questions from you.

I am sorry about that if my attitude has offended you; please forgive me and continue be my English teacher.

As to the OALD9th, I apologise for not explaining further. I mentioned the OALD, not to 'challenge' your authority (why would I do so when I have come to learn English?), but to show that dictionaries can't explain everything in detail, otherwise dictionary editors would have an even heavier burden on their shoulders.

As to your answer, my level of English as a non-native does not enable me to understand 'learn each phrase individually', which I think is quite idiomatic among natives, but not non-native speakers; do you mean WORK ON and WORK AT have differences that can only be learned by memorising and too subtle to explain in words?

Your apology is welcome, and is accepted.

To continue, now I've had a few days to think about this: it's difficult (and sometimes dangerous) to try to formulate a rule. Exceptions will always crop up.

I think it might be useful, in general, to think of WORKING ON to be about a single project.
example:
He's working on his fitness/ improving his manners/ his PhD/ his image/ his boatbuilding project.

and WORKING AT to be more general.

There is some overlap of course, and there are regional differences too.
As mentioned before, it's not a big mistake to swop (or switch) one for the other.

A few more thoughts on this:

example:

He's been working on his house for months. I think he's working at the plastering now.

also,

"Working at the plastering" = he's probably re-plastering a few rooms.
"working on the plastering" = the plastering already exists, and he's doing some repairs.

(PLASTER is a gypsum-based wall covering, applied wet.)

Okay are we all finished here? Is this any clearer now?

I like the second, which offers a good example of an interesting nuance.