Which of these words can be uncountable nouns : cucumber, onion, potato, carrot?
Could you give me examples with these words when they can be used as uncountable nouns?

Thank you in advance.
When you are speaking of the whole vegetable they are countable

Slice the carrot finely. Peel and quarter the potatoes.

When they have been prepared and are considered as substances they become uncountable.

I would like a plate of mashed potato (not mashed potatoes). Add the sliced carrot (regardless of how many you started with as whole carrots) to the soup.
Alex, Nona said that when you are talking about a whole vegetable, they are countable.
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Thanks, nona the brit.

Are these words used as countable or uncountable in following sentence?
"Could you buy some cucumber(s), onion(s), carrot(s) and potato(es)?"
 BarbaraPA's reply was promoted to an answer.
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