when do you know a word needs an accent when writing in Spanish?

accents can be used for questions, like Qué, Quién etc when the word ends in a vowel, n or s, the stress of the word should be on the second last syllable. if it is not, then it must have an accent.when the word is ending in a consonant (not n or s), the stress should be on the last syllable. if not, then it must carry an accent.some words have accents to tell two words spelt the same apart, e.g. si and sí.