I believe you should say "Can anyone explain to me when I can use the singular and plural of content?"

https://dictionary.cambridge.org/grammar/british-grammar/content-or-contents

Hmm, interesting. Thanks for the question, Val. In all those examples John provided it would be "content" (singular, uncountable) in Russian, and I, too, don't quite understand the fundamental difference between them. So, an essay is abstract enough to see its content as "content", and a book is not as abstract to see its chapters as "content", it's "contents"..? And what if you only have a half of onion in your fridge and nothing else, would that be enough to say "content" instead?

Jason did help you he said

"content = for singular items: as explained in the link using a piece of paper as an example"

CONTENTS="plural: as explained in the link using the individual chapters of a book as an example"

he used a different sentence structure to me, but he did give you the answer.

'The contents of the box of cereals may settle down to the bottom of the packet during travelling, but the contents are still the same weight as shown" = it may look like you do not have a full packet but the packet is filled by a machine".

"I'm sorry but I do not like the content of this discussion in front of children. Pleas refrain to talking about this when in adult company only"

"the content of mans combined knowledge, as not been written into one volume"

"The contents of my refrigerator/fridge says much about me"

"There is not much content in the essay you submitted as your homework"

Thanks Mohammed,

and you right about my question that it is written unclear.

@Jason,

You don't understand my question, but thanks for trying to help.