Your sentence (I've corrected some minor capitalisation and punctuation issues):
"Had" is the past tense of the verb "to have".
You correctly note that "Had" is the subject. This is a mentioned word, which is why there are quote marks. The predicate is the part of the sentence that tells you about the subject. The predicate is
is the past tense of the verb "to have"
The word "past" is an adjective, here it is modifying the noun "tense". Sentences don't have attributes, but an adjective gives an attribute to a noun. The phrase "past tense" is stable enough to be considered a compound, so you can consider "past tense" to be one word.
"of the word "have" is a prepositional phrase, and it modifies the noun phrase "past tense". It tells you which word's past tense you are discussing. Compare this with
This is a hair of my dog.
This [subject] is [verb] (a hair) [noun] (of my dog)[prep phrase]
The structure is broadly the same.