'Past Tense' should really be called 'past tenses' because you got yer
Simple Past Tense, [Myrtle hunted,]
and yer Past Progressive, [Myrtle was hunting,]
and yer Past Perfect, [Myrtle had hunted,]
and yer Past Progressive, (or Past Perfect Continuous,) [Myrtle had been hunting.]
And there may be some others, for all I know. All these verb tenses carefully define relationships between the particular bits of the past when stuff is happening. They are the 'home tense'.
But sometimes we step outside the home tenses and use Present Tense. Couple of reasons for doing this.
1) Dialog. Dialog talks past, present, or future or anything else you can think up. Dialog is just promiscuous when it comes to verb tense.
"I was late yesterday. I'm on time today. Tomorrow, I'll be early," Milly said.
2) Internal Monologue is always first person/present tense.
"I was late yesterday. I'm on time today. Tomorrow, I'll be early," Milly said, thinking, I can't afford all this confusion, in a distracted sorta way.
3) We slip into Present Tense to make certain general observations or state universal principles.
The narration that runs along and around and underneath the action and dialog is generally in Past Tense, of course.
Fr'instance here --
Jenny slid down the bank, yelping as she went. Even this early in the season the chuchimungas were vicious. Every one of them had sharp little spines that cut through the canvas of her trousers.
That is all in Past Tense. Past Tense is good. Doing Past Tense for this sort of narrative is never wrong. One can write for a whole long and glorious career and never handle tenses any other way.
But you can also inject Present Tense in the narrative to achieve special effects.
Jenny slid down the bank, yelping as she went.
The locals could have told her. Even this early in the season the chuchimungas are vicious as snakes. Every one of them bears a ring of sharp little spines capable of cutting through the canvas of even the heaviest trousers.
Lookit Dorothy Dunnett
(pause for DD fangirl squee)
injecting present tense:
Then the horn, blowing the right, told the hare had been found, and like split pulses the party tumbled apart.
A beaten hare, far from landmarks, forgets to run in a ring. A beaten hare runs uphill; and if she is old and shrewd and there is a fresh young hare at hand, she will clap and lie by the young on, an let her spring up first, is she will, so that the simpler braches, the pups and the addlepates, would bob and babble after the different scent.
It had happened here. But the older hare, rising, fled the meadow with half the company following, as the braches in the wood gave tongue after a different prey. (Dunnett, Queens' Play, quoted under Fair Use)
See how Dunnett is going along in her home tense, minding her own business. Then she pulls us into Present Tense. We step just a smidgen away from the scene in front of us. She speaks of universals, instead of the specifics of the scene.
Then she puts us back in the scene.
(Look! Look! See Dunnett write. Write, Dunnett, write.)
It works. This is Dunnett. Obviously, it works.
4) A byproduct of this two-step into and out of Past Tense is that we slow the pace.
Which is good if that's what you want to do.
Less good if you are in the middle of an action scene or an intense interpersonal bit or something else we do not want to slow or interrupt.
5) The journey from past to present to past again pulls the reader out of Deep POV. This is good if you have someplace you want to take the reader that involves moving out of Deep POV.
So you could maybe use it to switch POVs.
Here's a passage of Injected Present Tense where we're switching POVs.
It would be less heavy handed if I added in another couple hundred words.
Jenny drove her jeep down the rutted and steep mountain road. She was exhausted, barely able to see the road. The sun burned hot into her eyes. On every side the chuchimungas towered, menacing and somehow alien.
Central Funicia Province. The North Central Plateau. The badlands. The country of sullen, spiny cactus. The air is heavy with their green scent. They dominate the landscape. Every animal feeds from their stingy bounty or lives in the stunted yellow flowers among the spines or burrows under the creeping roots. Humans are unwelcome.
In the dirt, on the hilltop, hidden by brush, John Taylor watched the jeep approach. The field glasses were wearing circles around his eyes. Was this the right car? Finally?
"Hell." He felt sweat drip down his nose. It ran along the ridge of his binoculars and across the lenses.
6) And . . .when we step into a bit of Present Tense, we are somewhat freed from the lockstep of storytime.
We can return to the same scene and specify the time passed. The reader -- because she's moved into and out of the home tense -- can be coaxed to perceive a long passage of time.
7) Present Tense is a good place to make certain general observations and lay down a line of aphorism and suchlike.
The wash basin was cracked and provided a healthy living space for several species of mold. There's no place like home.
He picked up the revolver. It is never a good idea to leave loaded guns lying around. The door at the back of the machine shop was locked . . .
The howl of the vampyre panda sounded over the veldt. Trevor shivered. Pandas don't hunt while the sun is in the sky. That was the only thing that saved him. He went down on his hands and knees and tunneled deeper into the heart of the . . .
I admit I've used the 'aphorism' clause to slip into Present Tense in narrative.
There's idioms and catch phrases that need to be in present tense to sound right. 'What do women want?' 'Say it like it is.' 'It never rains but it pours.'
I've probably done that too.
8) Finally, there's Historical Present.
This happens when we have a First Person Past Tense situation. The First Person slips back and forth between Past Tense and Present. It's a recognition that First Person must take on some aspects of the 'now' in which that First Person exists. Third Person can be someone long ago and far away. First Person was there to write the story.
See Historical Present in Jim Butcher's writing.
I think we have Historical Present in Faulker's The Unvanquished -- which is in First Person Past Tense. In Unvanquished, Faulker slips into Present all the time. It is a delight to watch the Great Stylist do this. He has supreme control over his technique. I, who am not the second cousin of Faulker's cat, would not attempt such shenanigans.
He came toward the steps and began to mount, the sabre heavy and flat at his side. Then I began to smell it again, like each time he returned, like the day back in the spring when I rode up the drive standing in one of his stirrups that odor in his clothes and beard and flesh too which I believed was the smell of powder and glory, the elected victorious but know better now: know now to have been only the will to endure, a sardonic and even humorous declining of self-delusion which is not even kin to that optimism which believes that that which is about to happen to us can possibly be the worst which we can suffer. He mounted four of the steps, the sabre (that's how tall he actually was) striking against each one of the steps as he mounted, then he stopped and removed his hat. (copyright to William Faulkner, quoted under Fair Use.)
See how Faulkner shifts into and out of present tense to make that observation?
He does that shift because what he's saying doesn't fit well into past tense. He moves us out of the past-time narrative into a present moment when he wants to talk about this universal truth.
When I grow up I want to be Faulkner's cat.
Anyhow . . . all this shifting of tenses is skilled and difficult work that I do not plan to attempt myself but can admire from a distance. What I figger -- unless you have a knowledgeable foot on the clutch everybody's going to hear the gears grinding.