There are nine parts of speech in English grammar: noun, pronoun, verb, adverb, adjective, preposition, conjunction, interjection and determiners. Some writers and websites count only eight parts of speech and place determiner under category of adjectives. However, advance studies count determiner as a separate part of speech. These nine parts of speech indicate how the words function within a sentence. An individual word may function as more than one part of speech when used in various sentences. Study of part of speech is bit complex because one word may fall into many categories. Look at below examples to understand the concept.
(1) Ali loves singing.
(2) Ali is singing.
In above examples the word ‘singing’ is used twice but the function of the word ‘singing’ is separate in both sentences so this word is gerund noun in first sentence and verb in second sentence.
In the same way a word can be an adjective and an adverb at a same time. Actually the function of the word in a sentence decides about the word that in which category it falls.