In every narrative, you have - potentially - two time levels: the
time of the telling (tot) and the
time of the story (tos).
Normally, the tos comes before the tot; this fact is expressed through the past tense.
Joe took a knife from the drawer and cut a slice of bread from the loaf.
The effect, generally, is one of telling a story as it happened.
Now, you sometimes get present tense in narrative:
Joe takes a knife from the drawer and cuts a slice of bread from the loaf.
How do we interpret this?
Either (a) things are as they seem and events occur at the time of speaking, or (b) using the present tense to refer to past events.
(a) The effect is one of commentary. Like sport reporting:
There's Joe. He opens the drawer, yes, and there - what's he doing? - he takes a knife. A knife! Will he find the loaf? Yes, he's spotted it. There he goes. He cuts, cuts... SLICE!!! We have a slice!!!
Basically, the narrator is with the characters, you are not, but you have a live link to the narrator.
(b) If the present tense is used for the past tense, the effect is that of visualising a memory.
You want to know what happened? Joe walked into the kitchen, cut himself a slice of bread and left.
Details? What details do you need? Fine, just for you:
So, Joe walks into the kitchen. He's wearing a shabby t-shirt, because nobody's watching him but me, and I'm used to his shabby t-shirts. He opens the drawer. He's whistling. I think it's Swan Lake, but I don't know my classics as Joe does. Anyway, he opens the drawer, picks a knife, the middle one - he has three - a small one, a middle one, and a big one. He takes the knife from the drawer. With his left hand he grabs the loaf and...
What d'you mean, "What loaf?" There was a loaf on the kitchen table. Yes, he's messy. He'd left it out the night before, I suppose, so it was still there. Or he never bothered to put it away properly. How should I know? I didn't ask.
Now, a lot of present tense fiction is ambiguous between (a) and (b), and you know what? In many present tense stories it simply doesn't matter which one it is.
For a summary we have: Past tense is the tense of narration proper (of telling something that happened); present tense, on the other hand, is the tense of commentary, or imagination.
Going back to my "memory example" above, you'll notice that there's a switch in tense. First, past tense ("this is what happened"), then present tense ("I'm remembering as I go"), then again past tense ("I'm giving you background information; you interrupted me - I'm no longer in imagination mode").
Notice that - in a narrative - what's important is not to mess up the sequence of events.
YES: Joe takes/took a knife from the drawer and cuts/cut himself a slice of bread.
NO: Joe cuts/cut himself a slice of bread and takes/took a knife from the drawer.
This is obvious. Nobody makes that sort of mistake. But, see, you
can while you cannot have this
event sequence, you can have this narrative sequence if you like. You guessed it, you'll have to manipulate tense:
Joe cuts/cut himself a slice of bread, after he has/had taken a knife from the drawer.
Notice that, by changing the narrative sequence, you've taken the "knife-taking" out of the narrative proper and into background (it's now information on when he cuts bread). See how the perfect tenses signal information rather than narrative? Now, what if the "after-clause" had come before the main clause?
After he has/had taken a knife from the drawer, Joe cuts/cut himself a slice of bread.
While now "event-sequence" and "narrative-sequence" match again, the "knife-taking" is still in the background. It's not conceptualised as part of the story events, but as background information. This is true even without the "after":
Joe had taken a knife from the drawer. He cut himself a slice of bread.
What's the difference between "past-perfect - past" and the more common "past - past"? In the above you imagine Joe with the knife in his hand, already (plus you have the information that he took it from a drawer). In the normal sequence, you imagine the taking of the knife as an event in the story. Both can be good choices, but you'll use the "past - past" constellation more often.
This does have an editing implication, though:
If you find a sentence like the one above, you'll have to ask yourself why you took that odd sequence. You can, of course, edit it to "past - past". But if that's what you wanted to do, you'd probably have done it in the first place. You should
not treat this as a tense mistake. You can leave it as it is, if you think it works, but you can also - after figuring out what it is you were trying to do by this odd tense choice - address the point more clearly. So, if you wanted to create the image of Joe with a knife in his hand as an opening, for example, then you could do just that:
The knife was in his hand, but he didn't reach for the loaf immediately.
Note that general information can be framed as background within event-time, but also as background in story-telling time:
Joe owned three knives. One of those he took from the drawer.
Joe owns three knives. One of those he took from the drawer.
Framing general information as part of the telling-time doesn't automatically mean present tense, though:
Joe still owned three knives. On of those he took from the drawer.
Notice that it's not tense, here, but the word "still" that signals that the "owning knives" is framed as telling-time?
Normally, you have a "master tense" - i.e. the one that expresses your telling-event constellation. The default method is to progress through your events one by one, using that master tense. But you may want to escape into the story's background. Whenever you do this, it's possible that you change tense. These tense changes are
meaningful, even if they don't quite work. You (= your intuition) most likely had a purpose for the tense shift. If you figure out what you wanted to do, it's easier to edit. (Sometimes, a tense shift is just a mental hick-up; shifting from telling to imagination because something distracted you, for example. But it's always a good idea to wonder what you were trying to accomplish with the tense shift.)
An example. Here are the opening paragraphs from Walter Jon Williams' short story, "Incarnation Day":
Walter Jon Williams said:
It's your understanding and wisdom that makes me want to talk to you, Dr. Sam. About how Fritz met the Blue Lady, and what happened with Janis, and why her mother decided to kill her, and what became of all that. I need to get it sorted out, and for that I need a real friend. Which is you.
Janis is always making fun of me because I talk to an imaginary person. She makes even more fun of me because my imaginary friend is an English guy who died hundreds of years ago.
"You're wrong," I pointed out to her, "Dr. Samuel Johnson was a real person, so he's not imaginary. It's just my conversations with him that are imaginary."
I don't think Janis understands the distinction I'm trying to make.
Let's take it verb for verb. (I'm only talking about verbs that take a tense, here.)
is ('s), makes. moment of speaking, current situation, explanation
met, happened, decided, became. story time, plot summary as introduction
need, need, is. moment of speaking, current situation, explanation resumed
is making. generic time. Happens often. Note the progressive tense to emphasise repetition. Recurrant event, still going on at the moment of speaking.
talk. generic time. Reason for above "making fun of" explained. Notice that there is no emphasis of repetition, here. The narrator's own actions are framed as a simple statement of fact.
makes, is. same as above, only now Janis' actions are no longer emphasised for repition. Our narrator has already made her point.
died. past event framed by moment of speaking - subordinate to "is" above.
I'm leaving out the content of the speech, as this is a special circumstance (although the "was a real person, so he's not imaginary" is an interesting argument tense-wise.

)
pointed. Past event, framed in story time. This is the first instance of real narration, although it doesn't last long. It's about a specific event within the vague frame of "generic time" that's filled with "making fun of".
understands, 'm trying. Here the switch back to generic time (as opposed to "understood" and "was trying to make", which would still have been event time) underlines the idea that above event is exemplary. It's a singular event, but there are others like it, and they all have the same outcome. Janis doesn't understand, and our narrator keeps trying to make the point (this is why repetition is, this time, emphasised for the narrator, not Janis).
Eventually, the narration will slip into conventional past tense narrative, but there will always be interruptions framed by the "moment of speaking" (we're eavesdropping on our narrator's imaginary conversations with the real though dead and hence absent Dr. Samuel Johnson, after all). See how you can modulate tense for effect? A lot of writers do that quite automatically. Analysing (and thus editing) it is harder than writing it. Watch what you're doing, but don't regularise too much, or you run risk of depriving your stories of interesting word-tools.
Tense shifts, though often just slips, may work, and even if they don't they may constitute a failed attempt at technique that a simple "tense-alignment" might obscure rather than address. I'm aware that this is complicated, and I'm not at my height currently, but I'll hit "submit" anyway in the hope that I'm not
causing more confusion than I'm solving.