Particles in English grammar are function words that express grammatical relationships with other words. Function words perform definite grammatical functions but lack definite lexical meaning.
Particle is a grammatical function.
The grammatical form that can function as the particle in English grammar is the p-word.
The p-words of phrasal verbs, quasi-modal verbs, and some determiners function as particles.
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Hopper, Paul J. 1999. A short course in grammar. New York: W. W. Norton & Company.
Huddleston, Rodney. 1984. Introduction to the grammar of English. Cambridge: Cambridge University Press.