Why do “would” and “could” make questions polite?

Question

An excerpt of the article from thoughtco.com:

Key Words That Make Direct Questions More Polite
In informal situations, one could use the word “can” in a direct sentence. In the United States, “can” is considered to be incorrect for written English in particular because, in the past, it was not a word used when asking for something. Saying “May I have” instead of “Can I have” is preferred in the U.S. In the United Kingdom, the word is not frowned upon. Cambridge University publishes English teaching materials with the phrase “Can you lend me,” “Can I have,” etc.

In both countries, questions with “can” are made more polite by using “could:”

Excuse me, could you help me pick this up? Pardon me, could you help me? Pardon me, could you give me a hand? Could you explain this to me?

“Would” can also be used to make questions more polite:

Would you lend me a hand with the wash? Would you mind if I sat here? Would you let me borrow your pencil? Would you like something to eat?

“Would” in Oxford English Dictionary:

Expressing a polite request.

‘would you pour the wine, please?’
‘Would you please turn around?’
‘Would you mind clarifying your comment, Alison?’
‘Ruth, would you go with me to London?’

“Could” in Oxford English Dictionary:

Used in making suggestions or polite requests.
‘you could always ring him up’
‘could I use the phone?’
‘We had a letter to say they had not received the form and could I make a request for a new one.’
‘Where did you find the information that you used to write it, and could you suggest a few books for me?’
‘I would be grateful if any readers could tell me more.’
‘Is there a small heater you could recommend for when the icy weather returns?’

Why do “would” and “could” sound more polite than “will” and “can”?

‘Ruth, would you go with me to London?’ Why not: ‘Ruth, will you go with me to London?’

‘Could I use the phone? Why not: ‘Can I use the phone?

What tense or mood do “would” and “could” belong to?

Is it the subjunctive mood or the past tense, or something else?

From grammar-monster.com:

The subjunctive mood is the verb form used to explore a hypothetical situation (e.g., If I were you) or to express a wish, a demand, or a suggestion (e.g., I demand he be present).
…
If it were me, I'd go. (As this explores a hypothetical situation, was becomes were.)
I wish it were real. (As this expresses a wish, was becomes were.)
It is imperative that the game begin at once. (As this expresses a demand, begins becomes begin.)
I propose he work full time. (As this expresses a suggestion, works becomes work.)

Do requests with “would” and “could” seem hypothetical and therefore more polite? And with “will” and “can” they look more affirmative and therefore more rude?

‘Ruth, would you go with me to London?’ – seems hypothetical and therefore more polite.
‘Ruth, will you go with me to London?’ – seems affirmative and therefore more rude.

‘Could I use the phone?’ – seems hypothetical and therefore more polite.
‘Can I use the phone?’ – seems affirmative and therefore more rude.

It was my guess.

What are the reasons for using “would” and “could” instead of “will” and “can” to impart politeness to requests? Perhaps there is some psychological explanation for this. Maybe some ways of people’s thinking are reflected in it.

Answer 1

Would and could are used for imagined situations. Thus, when they are used in place of will and can, which express "something that is to happen" or a wish and possibility, they sound more polite; would and could imply that that which is being asked may or may not happen, regardless of whether it be possible or not, or it be desirable or not.

Merriam-Webster

History and Etymology for will

Verb (1)

Middle English (1st & 3rd singular present indicative), from Old English wille (infinitive wyllan); akin to Old High German wili (3rd singular present indicative) wills, Latin velle to wish, will

Merriam-Webster

History and Etymology for can

Verb (1)

Middle English (1st & 3rd singular present indicative), from Old English; akin to Old High German kan (1st & 3rd singular present indicative) know, am able, Old English cnāwan to know — more at KNOW

Answer 2

The bottom line here is that it is reducing a command to a request. "Pour me more wine" is a command, and "Would you pour me more wine?" is a request. Nobody likes to be ordered what to do, and so turning it into a request is considered more polite.

You might consider this. (Doesn't that sound nicer than me demanding that you: "Consider this.") I remember watching a TV show where a museum guard had to clear out a room of people. He approached someone and said "I'm going to have to ask you to leave." This is a curious phrase, don't you think? He didn't say "Get out." It is almost as if he was saying "In the near future I am going to have to tell you to leave, but if you do it now, of your own accord, then it'll be less awkward for us both, and less subservient for you."

So many of the niceties such as "could" or "would" are there to transform a demeaning demand into a more digestible request. Requesting is considerably more polite than ordering.

Answer 3

Both ‘can’ and ‘ could’ are modal verbs that refer to ‘a possibility’, ‘ability’ or ‘capacity’.

Difference between Can and Could

So what is the primary difference between both?

Well,

‘Can’ refers to a general truth or something that has a strong possibility.

‘Could’ refers to something that has a weak possibility, or something that might happen, but not necessarily a general truth.

(ibid)

This means, even if 'can' and 'could' both refer to possibility of something, the latter one seems more polite as it does not necessarily imply that the person must be able to do the task but rather more of an assumption that it might just be possible.

This holds true even for 'will' and 'would', the latter one when used makes the listener feel the there is quite a possibility that he/she be able to do the said task but not necessarily.