Interjection words are official parts of speech that do not have a number of grammatical features so that they can be considered independent: they do not have categories of numbers, gender, do not tend and do not change in cases and numbers. Yes, and the role in the proposals assigned to them is not the most important. Nevertheless, you cannot completely do without them, especially in spoken language.
The fact is that interjection is a part of speech that expresses a certain emotion without naming it, and in a different context the meaning can be different, even if the word is the same. In addition, they may express an incentive to action. Most researchers are inclined to believe that the so-called "polite", or "etiquette" words can also be attributed to this class.
Interjection is not a well-studied linguistic phenomenon. Nevertheless, they are divided into three fairly distinct categories: emotional, imperative and etiquette. The first category includes such interjections, examples of which immediately come to everyone's mind: “ah,” “oh,” “cheers,” and so on. The second category includes a variety of “hey”, “tsyts”, “kysh” and similar words. Labels include courtesy formulas - "hello", "goodbye", "sorry" and others.
Obviously, part of the words went into the category of interjections from independent parts of speech, therefore they are called derivatives. There are non-derivatives that seem simpler. Usually, nouns and verbs go into the category of service, but in theory, almost any word in one situation or another can go into the category "Interjection".
This phenomenon is more common in oral speech than in written, but the use of such words is also characteristic of fiction. Especially often they are used in combination with jargon and tracing words from foreign words. This is especially evident in adolescent environments. Globalization has introduced into the Russian language such words as “wow”, “okay” and a number of others. By the way, it is curious that interjection is not a universal combination of sounds for all languages. Usually they are similar, but quite often they nevertheless differ. For example, an imperative interjection calling for silence in Russian sounds like "ts-ss", in English - "hush", and in German - "pst". There is something similar in their sound, probably, in this case, initially it was onomatopoeia.
By the way, it is with him that interjections are confused. In fact, it’s quite easy to distinguish between them - onomatopoeia usually does not make any sense other than the image of a sound. That is, "replicas" of any animals, as well as words designed to show that a certain sound was heard (for example, "clap", "bang"), will relate to this category.
Another interesting point: when learning a foreign language, interjections are almost not given attention. Due to this circumstance (or a number of other reasons), even after a long stay in the country of the studied language, a person still continues to use emotional interjections in his native language. Another probable cause may be the nature of the origin of these sounds - they break out unconsciously, reflexively.
Interjections are extremely important in our lives. They are not always noticeable, but help to make speech more lively and emotional.