* Past participle: This is the most common usage, as in "The project has begun." Here, "begun" acts as a verb modifier, indicating a completed action in the past.
* Adjective: In some cases, "begun" can function as an adjective, describing something that has started. For example, "The begun task was left unfinished." Here, "begun" modifies "task."
* Noun: While less common, "begun" can be used as a noun referring to the act of starting something. For instance, "The begun was met with excitement." This usage is relatively formal and uncommon.
To determine the exact part of speech of "begun" in a sentence, you need to analyze its function within that sentence.