Tomorrow I will have eaten the cake you were talking about yesterday. On verb tenses.

Adam Callis recently asked, 

I’d love to hear your thoughts on using was vs. had been. I run into this a lot when I’m writing about a past event/memory in detail, and I feel like I often overthink it and use had been too much when I could just as well use was.

Example:

I had been walking that morning when I’d seen something that had caught my eye.

as  opposed to:

I was walking that morning when I saw something that caught my eye.


Adam is talking about the difference between simple past tense and past perfect tense. I mentioned a couple of weeks ago that if you are a native speaker of English, you probably get your grammar right at least 90% of the time. But one area where even native speakers get tangled up is verb tense, and especially the perfect tenses.
 

Simple past, present, and future tenses are straightforward enough: 

Simple past: I ate, you ate, he/she/it ate
Simple present: I eat, you eat, he/she/it eats
Simple future: I will eat, you will eat, he/she/it will eat

Nobody gets these tenses wrong. Sometimes, however, you need a little more precision and nuance than simple past, present, and future can give you. Consider these three sentences:

Sentence 1: I ran at 6:00 this morning.
Sentence 2: I was running at 6:00 this morning.
Sentence 3:  I had run at 6:00 this morning. 

Sentence 1 is in simple past tense. Unless you have some reason for being more precise, this is how you would probably describe your morning run. Maybe you mean you got out of bed at 6am and put on your running shoes and went running. Maybe you mean you left the house at 6am on your morning run. Or maybe you mean you were done and cooling down by 6am. (This last doesn't seem likely; most people who announce that they have gotten up early to run want credit for the earliest possible wake-up time. Still, the grammar allows for this possibility.)

Sentence 2 is in what we call past progressive tense. That -ing form tells you that at a given point in time, a person was or is or will be in the middle of some action. In Sentence 2 I tell you that as of 6am, I had already started running but I wasn't yet finished running.

Sentence 3 is in the past perfect tense. The perfect tenses indicate that at a given point in time, a person has completed or will have completed an action. Perfect here means complete.

For both the progressive and the perfect forms, there are past, present, and future tenses.

Past Progressive: I was running, you were running, he/she/it was running
Present Progressive: I am running, you are running, he/she/it is running
Future Progressive: I will be running, you will be running, he/she/it will be running

Past Perfect: I had run, you had run, he/she/it had run
Present Perfect: I have run, you have run, he/she/it has run
Future Perfect: I will have run, you will have run, he/she/it will have run

Today I'm not going to get into the technicalities of the four principal parts and the helping verbs that constitute these verb forms. You probably know how to create these forms instinctively, and if you don't, you can look them up on a grammar website. (I do, by the way, plan to get into these and many other technicalities in an online course called Grammar for Writers...watch this space for details.) But I will give you a couple of guidelines for when and how to use perfect and progressive verb forms.

Planting a flag on the timeline
When you use either the perfect or the progressive verb forms, you are planting a flag at a specific point on the timeline, then setting your verb tense with respect to that flag. You are saying, in effect, at this specific point in time (which may be in the past, the present, or the future), the subject of this sentence is either in the middle of an action or has completed an action.

For the simple tenses, the time-flag is optional. I can say

     I stole some leftovers out of the refrigerator,

or I can say

     I stole some leftovers out of the office refrigerator yesterday.

Either is fine. But if I'm going to say I had stolen leftovers, or I will have stolen leftovers, or I was stealing leftovers or I will be stealing leftovers, I need to provide a time-flag: 

I will be stealing leftovers from the office refrigerator tomorrow if I forget my lunch again. 

or

I had stolen some leftovers out of the office refrigerator before I realized that they belonged to the CEO.

   
As you may have noticed from that last example, planting a time-flag doesn't necessarily mean naming a time (6am or October 12 or day after tomorrow or 1492). You can also plant a time-flag by placing one action relative to another action. 

I was running this morning while you were still sawing logs.

I was thinking about you when you came through the door. 

Martha will be singing opera while you tend to the cash register.

When you have finished your broccoli, you can have some pie. 

I have eaten my broccoli, and now I want some pie.

And often a time-flag places an action relative to another action and also includes a specific time marker:

When you rolled out of bed at 6:00 this morning, I had already run, showered, and dressed.

One last word about time-flags: the present-perfect and present-progressive forms contain their own time-flag. By definition, the time-stamp for the present forms is the present. I am eating fried chicken is fine. You don't need to say As of this writing, I am eating fried chicken.

How much precision do you need?
When you use the perfect or the progressive tenses, you signal to your readers that they need to pay attention to some nuance of time. You're communicating that the reader needs to understand that this action was complete at a specific time, or it was ongoing at a specific time. If you aren't communicating some nuance of time, (this action was complete before this other action started, or this action was still going on when this other action started), don't ask your reader to decode a more complex verb form.

Let's return to Adam's examples from the beginning of this letter:  

a) I had been walking that morning when I’d seen something that had caught my eye.

Or:

b) I was walking that morning when I saw something that caught my eye.

I can't really think of a situation in which Sentence a) would be appropriate. When you've doubled up on past perfect, you're probably overthinking things. Neither of these verbs provides a time-flag for the other. 

On the other hand, look how the verb "saw" in sentence b) provides a time-flag for the past-progressive "I was walking." The seeing happens in a moment, and at that moment, the subject is in the middle of another action (walking). That time-flagging is even more apparent if you simplify "I saw something that caught my eye" to "something caught my eye."

     I was walking that morning when something caught my eye.

Or consider this variation, which adds a little more nuance:  

I had been walking for an hour when something caught my eye.

That verb "had been walking," by the way, is actually a past-perfect-progressive form--something that sounds very complex but which you are fully capable of handing. Notice here that we have made the grammar more complex, but we have added nuance that is more than worth the added complexity.

This week one of my online students wrote a piece about a harrowing journey across a rickety bridge in Eastern Europe. These two sentences demonstrate how tricky the past perfect can be:

If we turned around, we’d have to travel back across a narrow bridge that had no railings and was made of loose planks that had moved as we had gone across them the first time. I was pretty sure I had seen the tail end of a car that had plunged off the bridge into the ravine nose first.

These sentences are grammatically correct. But do you notice how distracting all those had forms become? had moved, had gone, had seen, had plunged. This is a moment when it is probably worth trading precision for simplicity, provided you can do so without introducing grammar errors. 

If we turned around, we’d have to travel back across a narrow bridge that had no railings and was made of loose planks that moved when we went across them the first time. I was pretty sure I saw the tail end of a car that had plunged off the bridge into the ravine nose first.

As I said, the original wasn't grammatically incorrect. But the grammar draws attention to itself. You don't want your readers to be thinking about grammar while you're trying to invite them into a scene.  Correct grammar is better than incorrect grammar. But what you really want is grammar that is invisible to the reader.