Here are some examples:
* Stronger (more strong)
* Faster (more fast)
* Taller (more tall)
* Slower (more slow)
However, it's important to note that -er is not always used to mean "more". It can also indicate a comparative adjective, comparing two things.
For example, "He is taller than his brother" doesn't simply mean "He is tall". It means he is more tall than his brother.
There are also other suffixes that can indicate "more" depending on the word and context, like -est for the superlative form (most) or -ier for certain words.