أحدهما

In a sentence

• رأيتُ طالبين، وأحدهما كان يحمل كتابًا.

  I saw two students, and one of them was carrying a book.

  Here, "أحدهما" is used to refer to one member of a specifically mentioned pair for emphasis and focus.

• اخترتُ مسارين، وأحدهما أقصر من الآخر.

  I chose two paths, and one of them is shorter than the other.

  In this case, "أحدهما" identifies one item from a set of two previously mentioned elements.

• كانا فريقين يلعبان، وأحدهما فاز بالمباراة.

  There were two teams playing, and one of them won the game.

  Here, "أحدهما" indicates one among the two subjects referred to in the sentence.