Translation:Sarah is on a diet; she only eats cabbage.
My sample was biased. Your second comment prompted me to widen my survey. The Scots say 'a diet' (I live in Scotland now), no Welsh input, the English and the Irish are ambivalent but the South Africans are quite happy with 'on diet' (I lived there for 50 years). I guess I will have to make yet another change to my English language usage.
'Sarah is on a diet and she only eats cabbage' what is wrong with that? Of course the extra 'end'. I guess this is a very typical mistake you do and very frustrating. As your proficiency increases, so does your speed, and you will be much more liable to add a ´missing´ word or miss a trivial one. Not much that can be done about it, to pay more attention to such trivialities, is, as indicated, somewhat counter-productive. Am I the only one to meet with such frustrations? It has nothing to do with your understanding and using French.
In what sense does the meaning of ´she is on a diet, she only eats cabbage' change drastically when you add an 'and' because it seems smoother in English? True in certain contexts the presence of a three-letter word will make a drastic change such as
Il ne mange pas que du chou
il ne mange que du chou
then you should pay attention. My point is that the purpose of all those exercises is to build up your reflexes so you do not have to think, just as you build up muscles doing push-ups. I was only presenting a source of frustration, to which there is little remedy save gritting your teeth, wondering if that was being shared.
I understood you to be talking about paying attention to "trivialities" in general, not this sentence in particular.
You will only be able to distinguish the important "trivialities" from the unimportant "trivialities" if you pay attention to the unimportant ones as well.
It is important to quickly and automatically sense meanings and express yourself, if you get into the habit of painstakingly spell out each word letter by letter, each sentence word by word, you will do very well on Duo indeed, I do not deny this, but it will hamper you in real life. But this is my opinion, which I may not share with everyone (maybe with no one?)
Particular about this sentence about Sarah being on a diet, but I took it to be representative of a more general phenomenon of innocent absentmindedness. Of course some acts of 'innocent' absentmindedness in life could have fatal consequences (such as when driving a car).
The placement of "que". But in this sentence there is only one choice.
The choice in English is effectively redundant. Although "she only eats cabbage" tells us that she does not drink the cabbage, we probably knew that already.
Although most English speakers will understand this as synonymous with "she eats only cabbage", it actually is not. It is technically incorrect (as a translation). And if she drinks kale smoothies then it is factually inaccurate as well.