Noun: When "subject" refers to the topic of a discussion, book, etc., it is a noun.
* Example: The subject of the lecture was the history of the Roman Empire.
Adjective: When "subject" describes something or someone under the control or power of another, it is an adjective.
* Example: The king was subject to the laws of the land.
It's important to consider the context in which "subject" is used to determine its part of speech.