“…Hence, these textbooks did not include sufficient information on the modal verbs. Emphatics as the second subcategory are used to reveal certainty (Szczyrbak, 2017) by using expressions like 'it is clear, clearly, obviously, obvious, in fact', and 'definitely'. According to Hyland and Tse (2004), boosters are effective linguistic expressions in strengthening the illocutionary force of speech acts (Holmes, 1982). Obviously, the function of boosters is revealing confidence in an assertion (Holmes, 1988;Crismore et al, 1993) or ending the debate and the possible alternative argument (Hyland, 1998;Gillaerts & Van de Velde, 2010): ''by closing down possible alternatives, boosters emphasize certainty and construct rapport by marking involvement with the topic and solidarity with an audience, taking a joint position against other voices'' (Hyland, 2005, p. 53).…”