I learn French from different sources and even when I finish course here I start over again… I am not sure that TU (as you sing) in French goes as it says in this course “t’es”…
Maybe I am wrong…
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I wager that the course (or this chapter) is about every-day-French, thus teaching colloquial variants; “t’es” is just short for “tu es”, so that’s perfectly fine in spoken French. You’re likely to see it quite frequently in movie sub-titles, too. 
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