I am having trouble distinguishing between the usage of cela,ceci and celui,ceux,celle,celles.
After a bit of research, it seems that 'cela' and 'ceci' are used to replace something indefinite that can't be masculine or feminine (an idea, situation etc.) but in the Duolingo lesson the sentences don't seem to follow this rule. For example:
-Ceci est une tomate -This (here) is a tomato
-Ceci est du lait -This (here) is milk
Since we are referring to something definite should these not be:
-Celle-ci est une tomate
-Celui-ci est du lait
Could anybody shed some light on the usage of each?
As I know it, "ceci est un/une" is equivalent to "c'est un/une", in that ceci is a contraction of "ce ici" - the "ici" just emphasizes that the "ce" means "this" and not, say, "that" or "it". It's like the verbal equivalent of pointing to the thing you're referring to.
"Cela" is similar except that it's a contraction of "ce là"; the "là" makes it clear that the "ce" means "that" instead of "this" or "it".
Neither of these can be used as an article, though, like "ce" alone can. You'd never say *ceci tomate for "this tomato"; these are all pronouns and, as such, require a verb: ceci est une tomate.
Incidentally, though, you can keep them as articles by simply moving the "ici" (shortened to "ci") or "là" to the end of the noun and adding a hyphen, e.g. cette tomate-ci or ce lait-là.
"Celui", though, is a bit different; it mean's "the one", indicating a particular one among many. Like "cela" and "ceci", this also is a pronoun, not an article, so you'd never say *celui tomate (wrong gender, anyway). "celui" is specifically the masculine singular form; "ceux" is the masc. plural, "celle" is the fem. sing., and "celles" is the fem. plural.
Just like "the one" in English, it doesn't actually make much sense to use "celui" or any of its variants alone. Imagine I told you, "the one is a tomato". ...what's "the one"? Which is why, similar to "ce", "celui" has "-ci "and "-là" suffixed to make it mean this one or that one - also like pointing to whatever you're referring to. Celle-ci est une tomate, therefore, means "this one is a tomato" (implying, perhaps, the other one is a pumpkin).
Hope this helps!
Hi, thanks for your comment, I still don't quite see the difference in usage between the two sets of pronouns. Are they just interchangeable?
Not quite... ceci = this, celui = this one
If you understand the difference between these in English, then you'll understand the difference between them in French.
It might help to imagine that ceci "this" would be an appropriate answer to qu'est-ce que "what": "Qu'est-ce que c'est? / Ceci est une tomate." ("What's this? / This is a tomato.")
Whereas celui "this one" would be an appropriate response to quel "which": "De ces légumes-ci, quelle est la tomate? / Celle est la tomate." ("Of these vegetables, which one is the tomato? / This one is the tomato.")