C'est une très bonne question et c'est difficile d'y répondre !
"ce" & "ça" are demonstrative pronouns equivalent to "it" and "this" in english
- "ce" is used with the verb "être"
ex : c'est bien = it's good ("c'est" is actually the contraction of "ce" + "est"
ce sera bien = it will be good (sera starts with a consonant so " c' " becomes "ce" again)
this is my friend = c'est mon ami (you can also translate it as "he is my friend" but in french you cannot say "il est mon ami", we consider that you show the person and says "this" as a demonstrative pronoun, and describe what or who "this" is. There is an exception with jobs though, you have to say "il est docteur")
- "cela" and "ceci" is used with all the other verbs => "ceci" = ce + ici (something close to you) and "cela" = ce+ là (something far from you)
But we don't really use when we speak, instead we use "ça" to replace "ceci" and "cela".
cela me plait => ça me plait
comment cela s'est passé => comment ça s'est passé
- "cela", "ceci" and "ça" can also be complement
ex: tu manges cela = you eat this (far)
tu manges ceci = you eat this (close)
tu manges ça = you eat this (spoken language / casual written language)
There are so many rules so my advice is:
- remember "ce" replaces "this" or "it" with "être"
- use "ça" for all the other verbs
- learn to recognize "ceci" and "cela" in written language but don't bother using them when you talk, we don't. Use "ça" instead.
It is a very vast subject, I'm not sure I covered everything, so if you have any question you can comment ;)