I think you are asking, Can I always form the comparative adjective by adding -er, and by extension the superlative by adding -est?
You can in many cases, but there are times when you cannot.
When you cannot or if in doubt, use the more, most constructions.
Here is an example of one you can't use the -er, -est endings:
beautiful
beautifuller

beautifullest

more beautiful

most beautiful
More and most constructions are always right, though they may sound a little strange at times depending on context and rhythm.
This dress is redder than that one.

<-- More commonly used
This dress is more red than that one.
Here is one where I'm uncertain.
This dress is purpler than that one.

Because I'm not sure if 'purpler' has entered the lexicon, I would say:
This dress is more purple than that one. Which still sounds a bit odd to me, possibly because the folks I hang around with don't go around comparing the degree of purple in things.
Orange Blossom