When the comparative form of an adjective changes, say from tall to taller, happy to happier, rich to richer, poor to poorer, do not use "more" before it.
- He is richer than me.
When the comparative form of an adjective doesn't change as in boring, enjoyable, common, difficult, tired, you use "more" before it.
- This year's paper is more difficult than the one before.
More examples:
- Hard work made him richer but more tired over the years.
- The new economic policies will make the rich richer and the poor poorer.
- Viral flu is more common during this time of the year.
