"i before e except after c except when it isn't"

The apparent exceptions of n and s are a result of their functions as third person verb marker and plural marker. Many Spanish words ending in consonants derive from words that had vowels that were dropped. The accent does not shift from what it would have been with the vowel still there. In the case of the final s, it is almost always a plural marker; thus boca, mouth is pronounced with the accent on the bo; the plural bocas is still pronounced with the accent on the bo. Similarly, the third person singular habla, he or she speaks, is pronounced with the accent on the ha; third person plural hablan, they speak, is also pronounced with the accent on the ha.