§ 337. The interjection is a part of speech characterized by the following features.
1. It expresses "emotions or will without naming them.
2. It has no grammatical categories, no stem-building
elements of its own and practically negative combinability.
3. It functions as a sentence-word or as a parenthetical
element.
§338. Some interjections are homonymous with other words: why', well!, now', here!, there!, come!, dear!, fiddlesticks!, etc. Others are not: hey!, hallo!, ah!, hurrah!, pshaw!, alas!, etc.
§ 339. Interjections, like other parts of speech, may be simple (hallo!, come!, dear!), derivative (goodness!), compound (fiddlesticks!) and composite (hang it!, dear me!).
§ 340. Semantically interjections are usually divided into two groups: emotional (oh!, bless us!) and imperative (hush!, come!1).
§ 341. A. I. Smirnitsky 2 thinks that interjections form a peculiar type of sentence, like that of the response-words yes and no, but differing from it in the distinct emotional colouring.
THE SEMI-NOTIONAL PARTS OF SPEECH3