tense vs past tense

tense and past tense both are nouns.

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

As nouns, past tense is a hyponym of tense; that is, past tense is a word with a more specific, narrower meaning than tense:

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

tense (adjective)	past 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

tense (verb)	past 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