When does the o-ending in the 3rd person pretérito indefinido get a tilde (like "oyó) and when doesn't it?
The regular preterite conjugations in the 3rd person singular (he, she, you formal) will always get a tilde as they will end in -o or -io, they will be stressed on that last syllable on the final -o, and that means you need to use the accent. That is just following the rules for accentuation.
habló, amó, bebió, comió, salió, fumó, trabajó...
However if the verb is monosyllabic, it doesn't take the accent:
dio, vio, lio, rio,...
The irregular preterite conjugations in the 3rd person singular where the stress doesn't fall on the last syllable but in the penuntimate syllable, there is no accent:
dijo, puso, hizo, anduvo, tuvo, quiso, estuvo...
There are also some completely irregular verbs in the preterite, like ser or ir, where there is no accent because of being monosyllabic words too:
I suggest to have a look at these kwiziq lessons:
Irregulares graves (no accents)
Where to stress a word in Spanish
Accents in Spanish
I hope it helps
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