In interrogative sentences, obviously the most basic is just simple command form: "Stop doing that". It tends to sound aggressive and blunt, and even impolite in formal settings.
One of the ways that this straightforward manner is attenuated is by the use of the word "please", softening the command to a request (though it can still be used for mandatory actions): "Please stop doing that" or "Stop doing that, please". This is soft enough for even the most formal of situations, interestingly. Young children are in fact taught to always say please with their interrogatives in order to be polite.
We also start to use the conditional as we get even more formal: "Could you stop doing that?", which is more formal in that it's very indirect; the asker is not asking whether the subject is literally capable of stopping, but rather requesting/mandating politely that the person stop.
Those can be combined, as well, as with one more tactic: the indicative syllogism. This is when the command is phrased in a very roundabout way, such as: "The way you do that is upsetting the children", which in context would be a request to stop. This one requires some logical ability, and subjects may not get the hint, but when executed well, it's very tactful and polite. The subject may not even realise you've asked them to stop!
Also, parents talking to children sometimes use indicative statements with "we". For example, a child might say to her friend, "Make sure to wash your hands", but the friend's parent might say "We wash our hands before dinner in this house". This is considered to be condescending and pejorative if done between adults, however, as it's traditionally reserved for obvious circumstances of a superior mandating to an inferior.
Combine all but that last one, and compare with the original:
- "Stop stepping on those plants!"
- "If those plants were to be crushed underfoot, they may never bloom, and the garden wouldn't look very pretty!"
To me, the difference in politeness is obvious.
