The rule which you have seen is not true. Spanish uses accents to show when a word's stress/pronunciation deviates from the regular rules.
Rules of Accent Placement
- The acute accent (´) is used to show that the word is stressed on that syllable, rather than where it would normally fall:
- callate [not a word] vs. cállate [be quiet, command]
- The acute accent (´) can also be used to clarify a word's meaning:
- de [from, preposition] vs. dé [give, command]
- este/a [this, adjective] vs. éste [this, pronoun] vs. esté/á [be, command]
- The dieresis (¨) is used to force a u to be pronounced after a g (güe, gwey; qüi, gwee):
- verguenza [not a word] vs. vergüenza [embarrassment, noun]
Commands and Gerunds (-ing Forms)
If using a command that has a pronound (me, te, se, nos, os, le, les, lo, la, los, las), and the pronoun is attached to the end of the verb, you must put an acute accent on the normally-stressed vowel:
- Callar : calla + te = cállate
- Llamar + te : llamando + te = llamándote
Rules of Word Stress
Word stress is determined as follows:
- If the word ends in a vowel (a, e, i, o, u) or n or s, the stress falls on the penultimate (second-to-last) syllable:
- If the word ens in any other consonant, the final (last) syllable is stressed:
- If the word is a noun whose singular form ends in an s or n, the plural form is stressed on the same syllable, but an accent must be used.
If the word is a command or gerund with pronouns attached to it, the stress falls on the same syllable as without pronouns. (N.B. an accent must be used to show this change.)
- Callar : calla + te = cállate
- Llamar + te : llamando + te = llamándote.
If the word carries an accent, the accent is retained in the extended forms unless the ending moves the stress to the same syllable as the accent. (N.B. in these cases the accent is dropped.)