Why do people so often confuse the words "than" and "then", but nobody ever confuses "tan" and "ten"?

There's probably at least tan reasons for that.

Because then and than have an extra letter.  How much can a brain absorb?

Ten and Tan are used in totally different ways .. then and than are used in more similar situations.  

I don't hear people mixing up long words (like "rhinoceros" and "hippopotamus", for example), so I don't see what point you are trying to make.

"then and than are used in more similar situations"

How are they used in similar situations? I can't see that myself. I can't think of any examples off the top of my head. Could you post a couple of examples, please? I would appreciate anything that would clarify this matter.

Why do you constantly assume that people make grammatical and spelling mistakes based on some logical decision?

Because I was educated that way. If my assumption is wrong, you tell me why else they do it.

"Then" and "than" are function words, "tan" and "ten" are content words. Content words have more salience.

Additionally, "than" and "then" (especially "than") are often pronounced with reduced vowels (as in common with function words) and the reduced vowels can erode their differences.

What foolish teacher taught you that people's mistakes are supposed to be logical?

If you really want to understand the evolution of language, and the rules of how it continuously changes, you need to study philology, and linguistics, of which I know very little. There are rules about how sounds change, and words change, and so on, but they are well beyond the scope of a casual discussion, or even basic grade school education. I suggest you make use of this amazing invention called "the Web", and maybe take some courses to learn more about it.

Reduced vowels? What do you mean? I explained the pronunciation in my own question. The only place where I could understand the confusion would be in Norfolk, where the regional accent makes "tan" sound like "ten". It might be excusable there due to the regional accent, but not anywhere else that I know of.

I don't think "function words" and "content words" make any difference to the pronunciation. That pertains to word usage, not pronunciation.

The "foolish teachers" didn't teach us that people's mistakes are supposed to be logical, they taught us that word usage is logical and from that, people's mistakes of that nature are inexplicable and inexcusable.

As for "evolution" of language, this isn't it. If anything it's "devolution" of language. Lol:)

Actually it does pertain to pronunciation. Consider a word like "for", which is often pronounced "fer" in rapid speech. Cf. the word "four" the number; this one is never pronounced with a reduced vowel, that's because it's a content word. Vowel reduction is a regular process of English in multi syllabic words, but in monosyllabic words it is rare outside of function words like "the", "of", "for", "than", etc.

First you say they taught you mistakes ARE based on logical decisions, then you say they didn't teach you that. Are you sure you paid attention in school, really?

LOL, mistakes are inexplicable, and inexcusable? Sounds like you went to school in something like a Nazi concentration camp. Yes, English grammar is pretty well defined these days. Mistakes are natural, and there are explanations, but as I said before, the explanations behind the mistakes are subtle, and much more complicated than the actual grammar itself. 

Call it what you want, but the same forces that drive "mistakes" in language, are likely the ones that drive its overall evolution. It's not a static, unchanging entity, nor should it be. I'm far more interested to hear what people have to say, than whether they made some small mistake in grammar or spelling. 

They taught us that word usage is logical, so from that I logically assumed that such errors are not logical. That logical deduction was my own, not something I learned from a teacher. Does that make logical sense to you, Rpf?

I'm far more interested in keeping the status quo as it was when I was in school than I am in hearing anything new people have to say. I pay far more attention to what people have to say when they are discussing bringing back the old ways.

All right, I'll consider a word like "for".

What did you do that for?

Does "for" sound like "fer" in that question? I think not, especially in the North, where vowels are pronounced clearly and distinctly. Now, can you give me an example (regional accents aside) in which the word "for" sounds more like "fer"?

That's because "for" is sentence final in that example, an inherently emphatic position, especially for a question. An example of when the vowel in "for" is reduced would be something like "tit for tat", where "for" functions more as part of a constituent phrase rather than in its usual role as a preposition outright. Now of course I'm going by the standard American dialect, the one that I am most familiar with. At the same time, this type of vowel reduction is inherent to casual speech, which is where confusion between "than" and "then" may occur in the first place. Enunciated speech would alleviate the confusion, but most people don't enunciate in casual conversations, which make up the majority of our exposure to speech. If the sentence "it's a lot worse than I remember" is spoke quickly, for example, the vowel in "than" will probably be pronounced as a schwa, which is the default English reduced vowel.

Yfel twêgen nu scearpðancol ðone as ðôht orgilde stæfcræft wiht bêga flitcræftlic hogung to lârhûs. Dôð lârsmið efen [spells] ûpgetêon wægn [rules] râd [English] ðætte ierfian [been] [codified] ofer hýðnes. [These] [rules] ârian bêgra inboren brûcung of ðone as mennisc met by reason of ðêode. Duguð snotorscipe râd [these] [rules] [is] sum inde ' right to own or occupy land or connected with its occupation nîedeslibban de [teased] ût fram from sê [apparatus] un−l¯æd [scientific] smêaung. Swâðêana wægn andweardlic lâd of reordian [has] nâ [required] ðone as benotian of flitcræft. Hîe efen [arises] [spontaneously]or prefixmennisc−licnes cynn.[

I] [don't] oncnâwan ðætte [fascination] wið healdung [things] [static] to [unchanging]. [Maybe] [it's] cyning gamolfeax wiht hâwian? meltan thither êow wênan attraction with pron ðe ic [should]ongêanfêran st¯ælan wægn gamolfeax [ways], twêonigend ðâ man weorðfulnes mâ lîðe, mæsse−ûhta [whatever], forhwý [don't] êow [insist] wîsdôm ðêah−hwæðere word−cwide oft oftor tô hwý unornlic Englisc?

Most of that is not English, so it has no bearing on this discussion.

"That's because "for" is sentence final in that example"

Right, I'll use a different example.

I wish my childhood could have lasted for ever.

In that sentence, "for" is still pronounced as "for", and the word "ever" is more emphatic. Are my examples making sense yet, or do you want me to post another one?

Andy, I'm done here. You're not interested in a discussion or in learning anything. You're interested in projecting and you're completely ignoring any of my own examples and explanations. Have a good day on your high horse. Congratulations on learning nothing from a question you asked. 

(I love this guy, folks.)
~

Wait outside of her apartment; she can't hide forever.

~

I say ippo and ryenoceros all the time. What's the prob?

~