grammar - Is だ (plain form of です) omittable?

After studying copula verbs だ and です in japanese grammar books by myself a couple of months ago, I was under the impresion that だ is omittable as it is implied by the context.

Moving on, I recently began reviewing na-adj and i-adj with a different book, and found out plain non-past form of i-adj are conjugated without だ while plain non-past na-adj are conjugated with だ.

Examples would be:

i-adj: この時計は大きい

na-adj: 彼女が好きだ

That being said, it だ only used with na-adj or can it be used with i-adj and ommitted whenever the speakers wants to?

Answer

だ is not the plain form of です. They're related, but you can't use だ everywhere you can use です, so calling one the plain form of the other doesn't work.

です has two functions:

1. 
   
   

   As a polite copula, similar to だ:

   
   

   
   

   りんごだ　→　りんごです　（noun）
   きれいだ　→　きれいです　（na-adj）

   
   

   
   


2. 
   

   As a politeness marker, following i-adjs:

   
   

   
   

   うつくしい　→　うつくしいです　（i-adj)

   
   
   

   
   

i-adjs form complete predicates on their own, and there's no need to add a copula to them:

＊うつくしいだ　　　←　i-adj+copula, ungrammatical
　うつくしいです　　←　i-adj+politeness marker, OK

There's another difference between the two forms. As copulas, だ and です inflect for tense:

りんごだった　→　りんごでした　（noun）
きれいだった　→　きれいでした　（na-adj）

But as a politeness marker, です does not inflect; the word before it does:

うつくしかった　→　うつくしかったです　（i-adj）

It's true that you can generally omit sentence-final だ in informal style. It's also true that you must omit sentence-final だ before certain particles such as か and さ. But when you say うつくしい without だ, that does not mean だ is omitted; it was never there in the first place, and it's ungrammatical to add it.