o in pret. ind. with tilde or without?

Question

When does the -o ending in the third person singular of the pretérito indefinido get a tilde en when does it not? P.e. "he spoke" = habló, but "she said" = dijo. Has it something to do with regular and irregular verbs?

Accepted Answer

Hola len
It's to do with how the word is pronounced each time and following the rules for accentuation. 
It is actually also true that verbs that have this type of irregularity in the preterite: dijo, puso, hizo, quiso..., because the stress on these words all fall on the penultimate syllable and they all end in a vowel (-o), they don't take the accent as, according to the rules for accents, they don't if that's the pattern.
Then you have regular verbs where you see the accent on the final -o: habló, bebió, comió, vivió, trabajó, cambió... - these are all stressed on the last syllable and end in a vowel, therefore you do need that accent. Bear in mind though, that not all regular verbs in the preterite have the accent because some of them are monosyllabic and they won't take the accent (rules again): dio, vio...
Here's some information about rules for accentuation in Spanish. 
I hope this clarifies it.
Saludos
Inma