This work deals with strong and weak forms of grammatical words in the English language. The theoretical part focuses on the function of weak forms in spoken English as well as the rules for their occurrence. Furthermore, it outlines different approaches to teaching weak forms in EFL context. The practical part of the work examines the pronunciation of the strong and weak form of the word that in the speech of advanced Czech learners of English and assesses the degree to which they reduce the strong form into its weak counterpart in required environments.