Question

1. The head is very important because it has your brain inside.

(What is the part of speech of 'inside'? Is it an adverb?)

2. The bag is 50% off.
(What is the meaing of 'off'? What other expressions can we use?)

3. The bag is 50% discounted.
(Is this the same as #2?)

Answer 1

In 1, I'd say "Your head..." since you used "your brain" later in the sentence. The word "inside" is an adverb.

In 2, the word "off" means less than the regular price. That is, the store has taken half of the price off the regular price.

I would rephrase 3 a little: "The bag is discounted 50%." That means the same thing as sentence 2.