Theory

When prefixes are added to a word, the spelling of the word doesn't change.
When a suffix is added to a word the spelling is often changed.
There are three main ways in which the spelling changes.

If a word ends in Y, it is often changed to I when a suffix is added.
For example:
HAPPY + NESS gives HAPPINESS.

If a word ends in a silent E, often the silent E is dropped when a suffix is added.
For example:
NOISE + Y gives NOISY.

If a word ends in a consonant, often the consonant is doubled when a suffix is added.
For example:
RUN + ER gives RUNNER.

Although the spelling of many words changes when a suffix is added, many words do not have their spelling changed.
For example:
WAIT + ER gives WAITER.

There are rules that explain when the spelling changes, but they are complicated and there are numerous exceptions.

The best way to know how to spell a word is to learn the spelling of each word individually.