Learn English – Most polite, not too formal way to remind one’s elder of something

Both uses of the past form are possible. Some grammarians speak of the past form as "remote", because it "removes" the verb to a distance which may be either temporal or social.

Hi, Maria, I'm calling because I wanted to take you out for dinner.

Here you use the past form to make want less demanding. The remoteness is social.

Sorry I couldn't track you down last night. I wanted to take you out to dinner.

Here the primary sense is clearly temporal remoteness. You could, however, work in a degree of social remoteness by also employing the progressive construction, which in this case would have no implication of imperfectivity - it would just be a further distancing device.

Sorry I couldn't track you down last night. I was wanting to take you out to dinner.

Note that this last construction could also be used in the first example:

Hi, Maria, I'm calling because I was wanting to take you out for dinner.

In this particular instance, however, the progressive construction would probably not be used because it clashes with the progressive calling in the main clause.

• Grammarians give this the Latin name horror aequi, "the widespread (and presumably universal) tendency to avoid the use of formally (near-)identical and (near-)adjacent grammatical elements or structures" (Rohdenburg).

You'd be more likely to use it in a context without a prior progressive:

Are you by any chance free tonight? I was wanting to take you out for dinner.

To express the "see-what-you're-missing" sense in English you would employ a construction with a stronger sense of intention. The past would be necessary in this case, rather than optional, because you would be speaking of a prior intention which no longer holds:

I was going to take you out for dinner, but I've changed my mind.

I've actually profitably made heavy use of the phrase "friendly reminder" in email and text. As in:

Friendly reminder: your appointment with me is at 4pm this afternoon.

or

Just a friendly reminder, your appointment with me is at 4pm this afternoon.

The other thing you can do, which I also do, is to skip the preamble about how they might have forgotten, and simply reiterate the information they might have forgotten -- or otherwise slip it into the conversation:

I look forward to seeing you at our 4pm appointment later today.

A related trick is to include a reminder, clarification or request about something else entirely which happens to include the stealth reminder for what you're really concerned about:

I wanted to let you know, the ceiling projector is out of order in the room in which we're meeting today at 4pm. I thought I should mention in case you were planning on using it.

Hey, would you happen to have some spare dry erase markers you could bring to our 4pm meeting today? The conference room was out when I was there earlier.

Another trick, one which I don't use personally, but have seen others use, is to pretend one might have forgotten, oneself, and ask the party you suspect of forgetting for confirmation:

I'm sorry to bother you, but I think I may have written our appointment time down wrong. When did we agree to meet?

Yet another approach I use with my sometimes flaky boss, regarding our 1-on-1 meetings, is to ask, as if no commitment had been made, whether something is true (typically via SMS which is the only way to get him when he's in other meetings):

Are we meeting at 4pm today?

The nice thing about this approach is it lets him save face if he forgot and made other plans: he can just write back "No, not this week" or "No, I have to debargle the overthrusters" or "No, let's meet next week". He doesn't have to admit he forgot. Also, it doesn't make me look like I'm nagging him, the way sending reminders might.