1. Subject of theoretical grammar. Analytic and synthetic word forms

Particles usually refer to the word (or, sometimes, phrase) immediately
following and give special prominence to the notion expressed by this word (or
phrase), or single it out in some other way, depending on the meaning of the
particle.
The question of the place of a particle in sentence structure remains
unsolved. The following three solutions are possible:
(1) a particle is a separate secondary member of the sentence, which should
be given a special name;
(2) a particle is an element in the part of the sentence which is formed by the
word (or phrase) to which the particle refers (thus the particle may be an element
of the subject, predicate, object, etc.);
(3) a particle neither makes up a special part of the sentence, nor is it an
element in any part of the sentence; it stands outside the structure of the sentence
and must be neglected when analysis of a sentence is given.
Each of these three views entails some difficulties and none of them can be
proved to be the correct one, so that the decision remains arbitrary.
5. The Interjection
Interjections have for a long time been an object of controversy. There has
been some doubt whether they are words of a definite language in the same sense
that nouns, verbs, etc. are, and whether they are not rather involuntary outcries, not
restricted to any given language but common to all human beings as biological
phenomena are.
Modern scholars consider interjections part of the word stock of a language
as much as other types of words. Interjections belonging to a certain language may
contain sounds foreign to other languages. Thus, for instance, the Russian
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interjection ax contains the consonant phoneme [x], which is not found in English,
etc.
Semantic features
The characteristic features which distinguish interjections from practically
all other words lie in a different sphere. The interjections, as distinct from nouns,
verbs, prepositions, etc., are not names of anything, but expressions of emotions.
Another characteristic feature of the meaning of interjections is, that while
some of them express quite definite meanings (for instance, alas can never express
the feeling of joy), other interjections seem to express merely feeling in general,
without being attached to some particular feeling. The interjection oh, for example,
may be used both when the speaker feels surprised and when he feels joyous, or
disappointed, or frightened, etc. The meaning of the interjection itself is thus very
vague.
Syntactic features
On the sentence level the function of interjections is a controversial matter.
The usual interpretation is that the interjection stands outside the structure of the
sentence. Another view is that it is syntactically a kind of parenthesis at least in
some cases. The controversy cannot be decided by objective investigation and the
answer only depends on what we mean by sentence structure on the one hand, and
by some element or other being outside the sentence structure, on the other.