красная

In a sentence

• Красная книга содержит информацию о редких животных.

  The Red Book contains information about rare animals.

  Here, "красная" is used as an adjective describing the book, denoting its name and emphasis as 'red.'

• На столе лежала красная ручка.

  A red pen was lying on the table.

  In this case, "красная" functions as an adjective describing the pen, specifying its color.

• Я люблю смотреть на красную закат.

  I love watching the red sunset.

  Here, "красную" is the feminine direct object form of "красный," describing 'закат' (sunset), referring to its red color due to the time of day.