Modal particles (Modalpartikel in German) are always uninflected words similar to grammatical particles in English which are also uninflected. However, English grammatical particles in fact include function words such as sentence connectors, sentence substitutes, conjunctions, interjections and even adverbs, which German particles do not do. They are entities standing out for their simpliciy in German, a language which is so much more inflected than English.
Modal particles are much purer in that their only function is that of reflecting the mood or attitude of the speaker or narrator. They do not refer back to what has been said previously but only what the speaker's attitude is to what has been said earlier. They do not stand in for whole sentences, and they do not exclaim when the speaker is in pain as interjections in English do.
Modal particles need to be learned in daily usage as there is practically no literal translation for each in itself other than that it helps form the phrase which on the whole conveys the way the speaker intends to sound, either to tone down opinions on the one hand or emphasize something on the other, to be conciliatory here or to give sharpness of expression there. More to come