[fatimajaved]

What is the rule for using an -er ending or a "more" beginning.
Ex: She is happier than I. v. She is more happy than I.

Please let me know.

[jjwoodhull]

The general rule is that er is used when the word is one or two syllables. More is used when the word is three or more syllables.

Happy - Happier, Happiest
Efficient - More Efficient, Most Efficient

[Wondergirl]

From bartleby.com --

There are two patterns for comparison of adjectives, the inflected and the periphrastic. The inflected pattern adds -er to the positive degree of the adjective to form the comparative degree: small becomes smaller, happy becomes happier. To form the superlative degree, it adds -est: smallest, happiest.

The periphrastic pattern uses the adverbial intensifiers more and most: the comparatives of beautiful and ostentatious are more beautiful and more ostentatious; the superlatives are most beautiful and most ostentatious.

The generalizations that seem to account for whether we choose the inflected pattern or the periphrastic are these:
(1) most one- and two-syllable adjectives use the inflected pattern;
(2) adjectives of three and more syllables almost always use the periphrastic;
(3) the higher the frequency of two-syllable adjectives, the more likely they are to inflect for comparison;
(4) the periphrastic more and most may on occasion be used with any one-syllable or high-frequency two-syllable adjective, e.g. more dear, most happy.