* Past participle of "have": This is its most common use, as in "I had a great time."
* Auxiliary verb: When used with another verb, it forms a perfect tense, as in "I had eaten already."
* Noun: In rare cases, it can be a noun, meaning "a brief period of time," as in "I don't have time for a had."
To determine the specific type of word "had" is, you need to look at the sentence it's used in.