past tense vs tense

past tense and tense both are nouns.

Word	Noun	Adjective	Verb	Adverb
past tense	Yes	No	No	No
tense	Yes	Yes	Yes	No

As nouns, tense is a hypernym of past tense; that is, tense is a word with a broader meaning than past tense:

past tense (noun)	tense (noun)
a verb tense that expresses actions or states in the past	a grammatical category of verbs used to express distinctions of time

past tense (adjective)	tense (adjective)
	taut or rigid; stretched tight
	pronounced with relatively tense tongue muscles (e.g., the vowel sound in `beat')
	in or of a state of physical or nervous tension

past tense (verb)	tense (verb)
	cause to be tense and uneasy or nervous or anxious
	become tense, nervous, or uneasy
	increase the tension on
	become stretched or tense or taut