Particles are perhaps the most frustrating part of Japanese grammar for
most English speakers. Most English speakers learning Japanese are aware
of verb tenses and conjugations, so the variety of them in Japanese does
not seem to surprise the learner. But particles are quite foreign to
most English speakers, confusing them to no end.

English uses prepositions to modify nouns and verbs. Prepositions such
as "to", "at", "by", "around", and "without" should be familiar to
anyone forced to learn the rudiments of English grammar in their school
days. Prepositions are members of a class of linguistic objects called
adpositions. The opposite of prepositions, postpositions

are usually found in languages which lack prepositions. In some
languages such as Finnish these postpositions have become glued to the
words they modify, over time resulting in a set of fixed
declensions for nouns and conjugations for verbs. Indeed,
some linguists theorize that all nominal declensions (and verbal
conjugations) derive ultimately from postpositions being suffixed to the
words they modify. Following this theory, a language which fossilizes
its postpositions into nominal declensions (a.k.a. cases)
eventually ends up needing a replacement set of adpositions and so,
perversely, produces prepositions to fill the gap.

Japanese has not gone so far as to completely fix its postpositions into
nominal declensions and verbal conjugations. Instead its postpositions
function as particles, small word-like units which modify the
meanings of the phrases to which they are attached. Particles do not
function as independent word units, they lack any meaning of their
own. Thus, a sentence like "が" cannot possibly have any sort of
meaning, either explicit or implied. It might be used as a prompt, one
speaker saying the particle to prompt another speaker to finish their
statement. But no meaning can be applied to the particle itself in any
context.

Particles used as prompts are actually fairly common. The particle で is
derived from the copula verb だ and is used to connect sentences
together. It works somewhat like English's 'and' or 'so'. This particle
is often heard alone when a listener is prompting a speaker to finish a
sentence which they have left incomplete.

学校にいって...
がっこう に いって
"I went to school and..."

で？
"And?"
で、鈴木君を見た。
で、すずき くん を みた
"And I saw Suzuki."

Other than this sort of usage particles are typically not used alone.

There is a very fine line between particles and verb (or adjective)
conjugations in Japanese. Oftentimes a conjugation will seem to be just
that until suddenly you notice that it is also attached to a noun in
some random exceptional situation. Or that a particle for some strange
reason seems to be only attachable to verbs and won't mate with anything
else. This is probably because modern Japanese is in the midst of a
change of emphasis from particles to conjugations, or vice
versa. Linguistic speculation has not provided any conclusive answer
yet. But suffice to say that you should expect particles and verb
conjugations to regularly blur the boundaries between parts of speech.