Learn English – Difference between “due to” and “on account of”

The two words have overlapping meanings, so there are contexts where either one could be used. However, there are certain constructs where one would be regarded as much better than the other.

I'm not sure if I can provide an exhaustive list of contexts where you should use might instead of could, or vice versa. Let's start with the example you provided, which is when either word is followed by not.

The phrase might not indicates a possibility that the contrary could happen. The word cannot suggests that the contrary won't happen. And the phrase could not suggests that the contrary wasn't able to happen.

So:

He might not show up.

means that he might show up, but we're starting to think he may not, while:

He could not show up.

suggests that, because of some obstacle, he was not able to make it.

Let's say we're supposed to meet Mike at 8 o'clock, and it's now 8:20. We have no idea where Mike is. I might say:

Let's not wait any longer. He might not turn up.

However, just before I finish my sentence, my cell phone rings. It's Mike! Mike tells me that his flight is delayed; the airplane is having mechanical problems, they haven't even left the runway yet. Mike is 500 miles away. Now, I say:

Let's not wait any longer. He cannot be here; he's stuck in Memphis.

Next, my friend and I leave, and we go to Ted's house. Ted sees that Mike his missing, and asks, "Wasn't Mike coming with you guys?" I might reply:

We waited for him, but he could not be here. His flight got delayed.

This is because, while might infers a possibility, could can infer a possibility, or an ability. Let's say you set a bar a meter off the ground, and ask me if I'm able to jump over it without touching it. I could say:

I might be able to do that. (it's possible)

But now you raise the bar two meters off the ground, and ask me if I can jump over it. I might say:

There is no way I could do that. (it's impossible)

However, there are times when the words could be used interchangeably, as you have sensed. For example, let's say we're leaving the house for a few days, and we're trying to decide whether or not to leave the furnace on. I might say:

Let's make sure the furnace gets turned on before we leave. We might get some cold weather.

or:

Let's make sure the furnace gets turned on before we leave. We could get some cold weather.

In that context, the two phrases mean essentially the same thing.

Now, an interesting example for the advanced reader. Consider these two statements:

The repairman might come today, but he could come tomorrow.
The repairman could come today, but he might come tomorrow.

Is there a difference? I'd say that both of them mean that the repairman is supposed to come either today or tomorrow, although there's a little uncertainty as to which day he will arrive. But does one of them suggest a higher probability one way or the other? I don't think so, at least not reliably.

The simplest way to think about these two terms is perhaps,

• to think of a user account as the user himself or herself, and
• to think of his or her profile as information about that user.

Sometimes the two terms are somewhat overlapped.

From Wikipedia,

A user is a person who uses a computer or network service. A user often has a user account and is identified by a username (also user name). Other terms for username include login name, screen name (also screenname), nickname (also nick), or handle, ...

A user profile is a visual display of personal data associated with a specific user, or a customized desktop environment. A profile refers therefore to the explicit digital representation of a person's identity. A user profile can also be considered as the computer representation of a user model. A profile can be used to store the description of the characteristics of person. This information can be exploited by systems taking into account the persons' characteristics and preferences.

From Microsoft,
^{(this is specific to Windows operating system, but the concept is similar to user account and user profile in web programming)}

A user account is a collection of information that tells Windows which files and folders you can access, what changes you can make to the computer, and your personal preferences, such as your desktop background or screen saver. User accounts let you share a computer with several people, while having your own files and settings. Each person accesses his or her user account with a user name and password.

Your user profile is a collection of settings that make the computer look and work the way you want it to. It contains your settings for desktop backgrounds, screen savers, pointer preferences, sound settings, and other features. User profiles ensure that your personal preferences are used whenever you log on to Windows.

A user profile is different from a user account, which you use to log on to Windows. Each user account has at least one user profile associated with it.

^{(emphasis added)}