чай

In a sentence

• Я люблю пить зеленый чай утром.

  I like drinking green tea in the morning.

  In this sentence, 'чай' refers to the beverage tea (green tea specifically).

• Мы попьем чай после обеда.

  We will have some tea after lunch.

  Here, 'чай' is used to denote the action of drinking tea as a social activity after a meal.

• На полке лежит коробка чая.

  There is a box of tea on the shelf.

  In this case, 'чая' is the genitive form of 'чай,' indicating possession of or relating to the box's contents (tea).