I'm always confused by these two words. Like, when do I say 'he spoke to him' and when do I say 'he spoke to himself'? And with reflexives, like 'he hurt himself' versus... I don't even know which one's right? Can someone explain this in a simple way?
The 'se' is always unstressed (no accent) and shows up in reflexive actions where the subject does something to themselves - "he hurt himself," "she combs her hair." Now 'si' is stressed (with emphasis, usually comes after a preposition) and is like an independent pronoun - "he spoke to himself," "brought it with him." A practical tip that works really well is try replacing it with the person's name: if you can say "he hurt himself" by switching it to "he hurt João," then it's 'se'! If it only works after a preposition like "spoke to himself" = "spoke to João," then it's 'si'. In everyday use, 'se' is way more common than 'si', so if you're unsure in a reflexive context, bet on 'se' and you'll hardly ever get it wrong.
The confusion is normal because 'se' and 'si' work in different contexts. 'Se' is a reflexive or reciprocal pronoun - it's what you use when the action comes back to whoever does it. Like "he hurt himself" (he hurt himself), "she combed her hair" (she combed herself), "they kissed each other" (one kissed the other). Here 'se' is always the unstressed form, meaning it goes right next to the verb with no preposition.
Now 'si' is a stressed pronoun that appears after a preposition. So it's "he spoke to himself" (to himself), "she bought this for herself", "he locked himself in himself". The big difference: 'se' comes with no preposition and stuck to the verb; 'si' always comes with a preposition (to, of, in, with, by, etc.). You'd never say "he to se hurt himself" - that sounds weird. It's either "he hurt himself" or "he spoke to himself" if you want to be clear.
One tip: if there's a preposition, it's 'si'. If there's no preposition and the verb is reflexive, it's 'se'. Actually, when you're really confused, think of "to himself" or "to himself" - the 'si' never shows up without that help from a preposition or the word "own". 'Se' is the quick form, the one you put before the verb or right after, depending on how the sentence is structured.
I picked up on it over time, but the easiest way to think about it is: 'se' is always the reflexive pronoun that sticks to the verb ("he hurt himself", "she combs her hair"), while 'si' is a stressed pronoun that comes after a preposition and has more emphasis ("he spoke to himself", "that's for you"). In practice, you almost never use 'si' in everyday speech - 'se' handles 99% of situations.
Don't fall for the trick of thinking 'si' is used for reflexive action - it never plays that role. 'Se' is your real reflexive, the one that shows up stuck to the verb ("he hurt himself," "she combs her hair"), and it also works in impersonal sentences like "you eat well here." 'Si' is different: it's a tonic pronoun (stressed, pronounced with force) that only appears after a preposition, and it's used when you want to emphasize that the action comes back to the same person - "he talks with himself" is quite different from "he talks with si," where the 'si' is there saying "the action comes back to the subject, see?" The confusion happens because in spoken Portuguese we rarely use 'si' (it's more of a European Portuguese or formal writing thing), but in formal writing you need to know: 'se' is an unstressed reflexive, 'si' is a stressed reflexive that comes after a preposition.
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