Tea and Coffee are the goods that can be considered substitutes.
Substitutes goods are goods where one can substitute the other in its absence.
If you don't have coffee but have tea, then, you can drink tea in lieu of coffee and vice versa.
Pen and paper, car and petrol, bread and butter are all complementary goods. One good must be present so the other good can be put to use. The increase in demand of the any one of the complementary goods will also increase the supply of the the other good.