Consider the following reference:
If there isn't a written accent mark, every Spanish word that ends in a vowel, an "-n," or an "-s" has the stress fall on the second-to-last syllable (the "penultimate" syllable)
However, if this rule is so, why is the "i" accented in the majority (if not all) of verbs when they are conjugated into the conditional tense. For example:
Mostaría
Echaría