In grammar the term particle (abbreviated PTCL) has a traditional meaning, as a part of speech that cannot be inflected, and a modern meaning, as a function word associated with another word or phrase to impart meaning.
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Modern meaning
In modern grammar, a particle is a function word that must be associated with another word or phrase to impart meaning, i.e., does not have its own lexical definition. On this definition, particles are a separate part of speech and are distinct from other classes of function words, such as articles, prepositions, conjunctions and adverbs. Languages vary widely in how much they use particles, some using them extensively and others more commonly using alternative devices such as prefixes/suffixes, inflection, auxiliary verbs and word order. Particles are typically words that encode grammatical categories (such as negation, mood, tense, or case), clitics, or fillers or (oral) discourse markers such as well, um, etc. Particles are never inflected.
Related concepts and ambiguities
Depending on context, the meaning of the term may overlap with concepts such as morpheme, marker, or even adverb as in English phrasal verbs such as out in get out. Under a strict definition, in which a particle must be uninflected, English deictics like this and that would not be classed as such (since they have plurals and are therefore inflected), and neither would Romance articles (since they are inflected for number and gender).
This assumes that any function word incapable of inflection is by definition a particle. However, this conflicts with the above statement that particles have no specific lexical function per se, since non-inflecting words that function as articles, prepositions, conjunctions, interjections have a clear lexical function. This disappears if particles are taken to be a separate class of words, where one characteristic (which they share with some words of other classes) is that they do not inflect.
In English
Particle is a somewhat nebulous term for a variety of small words that do not conveniently fit into other classes of words. The Concise Oxford Companion to the English Language defines a particle as a "word that does not change its form through inflection and does not fit easily into the established system of parts of speech". The term includes the "adverbial particles" like up or out in verbal idioms (phrasal or prepositional verbs) such as "look up" or "knock out"; it is also used to include the "infinitival particle" to, the "negative particle" not, the "imperative particles" do and let, and sometimes "pragmatic particles" like oh and well. Another example is the word "so" in the adverb "so far".
Afrikaans
The following particles can be considered the most prominent in Afrikaans:
Chinese
In Chinese, particles are one of two major word classes. The other class includes noun, verbs and adjectives. Linguists do not agree on whether or not Chinese pronouns and adverbs should be classified as particles.
German
A German modal particle serves no necessary syntactical function, but expresses the speaker's attitude towards the utterance. Modal particles include ja, halt, doch, aber, denn, schon and others. Some of these also appear in non-particle forms. Aber, for example, is also the conjunction but. In Er ist Amerikaner, aber er spricht gut Deutsch, "He is American, but he speaks good German," aber is a conjunction connecting two sentences. But in Er spricht aber gut Deutsch!, the aber is a particle, with the sentence perhaps best translated as "What good German he speaks!" The particles appear more often in relaxed spoken and casually written registers of German.
Japanese and Korean
The term particle is often used in descriptions of Japanese and Korean, where they are used to mark nouns according to their case or their role (subject, object, complement, or topic) in a sentence or clause. These particles may function as endings and therefore as bound morphemes rather than independent words, in particular in Old Japanese. Some of these particles are best analysed as case markers and some as postpositions. There are sentence-tagging particles such as Japanese and Chinese question markers.
Latin
In Latin, particles are the adverb, the preposition, the conjunction and the interjection (a word that has emotion).
Polynesian languages
Polynesian languages are almost devoid of inflection, and use particles extensively to indicate mood, tense, and case. Suggs, discussing the deciphering of the rongorongo script of Easter Island, describes them as all-important. In Māori for example, the versatile particle "e" can signal the imperative mood, the vocative case, the future tense, or the subject of a sentence formed with most passive verbs. The particle "i" signals the past imperfect tense, the object of a transitive verb or the subject of a sentence formed with "neuter verbs" (a form of passive verb), as well as the prepositions in, at and from.
Tokelauan
In Tokelauan, ia is used when describing personal names, month names, and nouns used to describe a collaborative group of people participating in something together. It also can be used when a verb does not directly precede a pronoun to describe said pronouns. Its use for pronouns is optional but mostly in this way. Ia cannot be used if the noun it is describing follows any of the prepositions e, o, a, or ko. A couple of the other ways unrelated to what is listed above that ia is used is when preceding a locative or place name. However, if ia is being used in this fashion, the locative or place name must be the subject of the sentence. Another particle in Tokelauan is a, or sometimes ā. This article is used before a person’s name as well as the names of months and the particle a te is used before pronouns when these instances are following the prepositions i or ki. Ia te is a particle used if following the preposition mai.
Thai
Thai also has particles.