"ce" is a demonstrative (before a noun or a verb: ce livre, ce qui, c'est). "se" is a reflexive pronoun that goes with a pronominal verb (se laver, se lever, s'amuser).
If you can replace it with "cela" or "cet", it is "ce". If it comes before a pronominal verb, it is "se".
Examples
Ce matin, il fait beau.
Key word(s): ce
"Cet matin" works (demonstrative before a noun).
Il se lave les mains.
Key word(s): se
Pronominal verb "se laver".
C'est une bonne idée.
Key word(s): c'
"Cela est une bonne idée" works.
Elle se promène dans le parc.
Key word(s): se
Pronominal verb "se promener".
Ce que tu dis est vrai.
Key word(s): ce
"Cela que tu dis" works.
Common mistakes
Writing "se matin" (using the pronoun instead of the demonstrative).
Writing "il ce lave" (using the demonstrative instead of the reflexive pronoun).
Confusing "c'est" and "s'est".
Test yourself
___ livre est intéressant.
"Cet livre" works, so this is the demonstrative.
Elle ___ réveille tôt.
Pronominal verb "se réveiller".
___ qui compte, c'est l'effort.
"Cela qui compte" works.
Tips to remember
"se" always comes before a verb.
"ce" comes before a noun or in "ce qui", "ce que", "c'est".
For "c'est" vs "s'est": "cela est" = c'est, "il s'est levé" = pronominal.
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