THIS ONE and THAT ONE - specifics and non-specifics - see also WHICH 01

When you're referring to a *specific* item (eg 'this one' or 'that one') - where the item itself is not *named* in what you are saying - you would use the 'celui', 'celle', 'ceux', 'celles' series. Note that the feminine plural is different from the masculine plural.

celui

this/that one (m)

celle

this/that one (f)

ceux

these/those ones

celles

these/those ones (f)

[Note that 'ceux' is used for 'masculine plural' but also for 'm+f plural', whereas 'celles' is used for feminine plural *only*.]

To be more specific (to actually specify 'this one' or 'that one'), '-ci' or '-là' are added, as per WHICH 01.

celui-ci	this one (m)		celle-ci	this one (f)		ceux-ci	these ones		celles-ci	these ones (f)
celui-là	that one (m)		celle-là	that one (f)		ceux-là	those ones		celles-là	those ones (f)

And to be less specific, ie not to refer to 'this cat' or 'that cat' or even 'this one' or 'that one', but simply 'this' or 'that' (as in "this is hard work", or "that's unbelievable") use 'ceci' for 'this', and 'cela' for 'that'. There's a compressed form of "cela" which is simply "ça", which is much more often used in general conversation.