"I have that tea."
Translation:Ja mam tamtą herbatę.
I had this question as well. The declensions of ten and tamten are so similar that the first time I learned them I though they were exactly the same. However, there's a difference, the accusative feminine. It is tĘ for this, but tamtĄ for that. Otherwise it would be too boring, I guess.
"tamta" (tamta herbata) is Nominative, "tamtą" (tamtą herbatę) is Accusative.
Nominative is used mostly for the subject of the sentence (Tamta herbata jest smaczna), Accusative is used for the direct object of the sentence (as here).
I'll let myself assume (from your nickname) that you understand Russian, so that would be "Тот чай - вкусный" vs "Я имею тот чай", Wait, that's not the best example as it looks identically here... OK, "Та вода - вкусная" vs "Я имею ту воду". And yeah, "У меня та вода" would be more natural but wouldn't show anything ;)