• nouns
• verbs: without a verb a group of words cannot be a clause or sentence.
  • auxiliary verbs
  • modal auxiliaries
• adjectives
• adverbs
• pronouns
• prepositions
• conjunctions
• determiners
  • quantifiers, numerals
• interjections

https://en.oxforddictionaries.com/grammar/word-classes-or-parts-of-speech http://dictionary.cambridge.org/grammar/british-grammar/about-words-clauses-and-sentences/word-classes-and-phrase-classes