Substitute could be an adjective or a noun.
For example,
"The substitute teacher" uses subtitute to describe what kind of teacher.
"The substitute passed out the homework" uses substitute as a noun by itself
A word such as a number or article or possessive adjective that determines or limits the meaning of a noun phrase. For example their in their black cat. Other articles are 'the, a and an' or any words that might substitute for them - yours their, some and each
You substitute the variable for its value. Or you substitute the variables for each of the values.
Cautious IS an adjective. An adjective is an action!
The word "it" is not an adjective (it is a pronoun). A word is an adjective if it modifies (defines, characterizes) a noun or pronoun. The big tent - big is an adjective He is tall - tall is an adjective This key - this (while arguably called a determiner) is a demonstrative adjective
probable is an adjective
An adjective modifies a noun or noun substitute by providing more information about its characteristics or qualities.
The adjective is ambiguous.
The word is manifest (in the adjective sense).
The adjective is indivisible.
The adjective would be invincible. (unbeatable)
The adjective is probably "cushy."
The adjective could be obsolete, outmoded, or superseded.
The adjective could be moral, ethical, honorable, or righteous.
An adjective meaning free from error could be flawless or perfect.
The adjective for such speech is "heartfelt." The person may be "earnest" or "sincere."
The term might be skeptic. Another adjective for skeptical is suspicious.
The word "makeshift" can function as both a noun and an adjective. As a noun, it refers to something used as a temporary substitute. As an adjective, it describes something that is improvised or temporary.