فإن

In a sentence

• قال المعلم: فإن كنت مستعدًا، فلنبدأ الدرس.

  The teacher said: If you're ready, let's start the lesson.

  In this sentence, "فإن" serves to introduce a conditional clause ("if you are ready") with an emphasis implied by the conjunction "ف-" (for).

• نظر إلى الساعة وقال: فإن الوقت يمضي بسرعة.

  He looked at the clock and said: Indeed, time passes quickly.

  Here, "فإن" functions to provide a segue, attaching emphasis to the statement about time passing quickly.

• كتبت الرسالة، فإن الرد لم يتأخر.

  I wrote the letter, therefore the reply did not delay.

  In this case, "فإن" conveys a causal relation, linking the act of writing the letter to the swift reply.