Care to Explain: The Function of German Modal Particles
First of all, I haven't really ever read an account that describes the practical everyday function of German modal particles very well. There is something unique about this type of word, as it doesn't exist in English. This is my own attempt at giving you an idea of what modal particles are used for in the German language. I'm a native speaker and a writer, and that makes me want to look closer at the phenomenon, but I'm not a linguist or a German professor. I won't approach the subject from a grammatical or linguistic standpoint here.
While you can read everywhere that modal particles are typically used in informal contexts, and that they do not alter the content of any sentence significantly, that does not mean they are unimportant. Sure, the more formal the context gets, the less likely it is that you'll find many modal particles. But in a speaking context it is rare that they're ever completely absent, and that underlines their significance. For example, due to their "humanizing" tone, they are commonly used by politicians in speeches, press statements, and interviews.
That element of humanizing communication is perhaps the most important thing you will have to understand about modal particles. Without them, many phrases sound plain, robotic and awkward in social situations. In English it is normal and straightforward to ask somebody, "Where is the bus stop?". In German, the direct translation "Wo ist die Bushaltestelle?" also sounds straightforward when you are reading it, but if you are actually in the situation to pose that question, it can make you look a little bit like The Terminator.
Being Informally Nice
Recognize that this is not about formal niceties, as in, saying please! This is about a much more subtle, almost tacit form of situational appropriateness. There is an English phrase called "Would you care to explain?" It implies that explaining your situation is a form of caring about the people you interact with. German modal particles tackle exactly this informal way of showing that you care. The idea is: Give the listener a minimal indication as to why you are saying this to them, and your sentence, your situation, and you yourself all become much more relatable.
Now, if you could explain with one uncomplicated word that it only just occured to you that you need to find the bus stop ("Wo ist eigentlich die Bushaltestelle?"), that you did not expect not to find the bus stop ("Wo ist denn die Bushaltestelle?), or that it is much harder to find than you thought ("Wo ist bloß die Bushaltestelle?"), wouldn't you seem a lot less robotic to the person you're asking?
You can still translate these variations of the same sentence into English, but the English version starts sounding funny once you combine several modal particles. What, for instance, if you both did not expect to not find the bus stop and have been looking for it for some time? In English, a phrase like
"Where on earth, then, is the bus stop?"
sounds complicated, old-timey, over the top, and far from intuitive. It sounds simple and natural with modal particles, especially as you won't have to alter the sentence structure at all - just add one particle on top of the other: "Wo ist denn bloß die Bushaltestelle?" Definitions of modal particles typically fail to highlight that it is actually most common to put several of them in a sentence, rather than one. In more complex situations than the bus stop scenario, you can stack three or more modal particles on top of each other.
Imagine then, that with only about a dozen words you'd have to learn you could improve literally almost any social interaction you are having in the same way. Just combine the right number of modal particles, and you are showing that you care, by explaining where your action or your feeling - be it anger, enthusiasm or confusion - is coming from. These words are so opaque, so flexible and universal, so unobtrusive, so spontaneous and intuitive that everybody who knows the language barely notices they are using and hearing them.
Wouldn't you want to have those types of magic words in English as well? It seems a little absurd in this context that German is the language with the cliché of coldness, harshness and technicality attached to it.