Parallelism in tenses

[Yash VardhanS404]

Hi

I came across this post from Magoosh
[redacted]
which states two clauses using different tenses can be parallel to each other. Can you explain the reason behind it ?

[RonPurewal]

tense has nothing at all to do with grammar; it's purely a function of meaning. for purposes of grammar/mechanics, ALL verbs are equivalent.

in other words, in an appropriate context just about any verb could be parallel to just about any other verb.

in fact, it's quite easy to create examples in which...
...it's clear that parallel verbs need different tenses,
...it would be ridiculous to put them in the same tense.
e.g., My cousin joined the Air Force in 1989 and will be discharged in four years.

[RonPurewal]

incidentally, all of the above observations are also true for active/passive. that, too, is purely a function of context/meaning; grammatically the two are equivalent.

conveniently, the same sentence i wrote above works as an illustration of this, too.

[aflaamM589]

Conversely, can you tell under what conditions the tenses in parallel structures have to be parallel?
One i know is, if there are simultaneous actions then tenses need to be same.
Are there any other conditions?
Best

[RonPurewal]

verb tenses are determined entirely by context, 100% of the time.

if the context makes it clear that the two actions are happening in the same timeframe, then the two verbs should be in the same tense. there's no "rule" for this.