بها

In a sentence

• أحب الطائرة الكبيرة لأنها تحتوي على مساحة واسعة، وأسافر بها دائمًا.

  I love the big airplane because it has spacious room, and I always travel with it.

  Here, 'بها' is used to indicate the means by which the action of 'travel' is performed – specifying the mode of travel being 'with it' (the airplane).

• وجدت كتابًا قديمًا في المكتبة وأعجبت كثيرًا بها وموضوعها.

  I found an old book in the library and was very impressed by it and its subject.

  In this sentence, 'بها' is employed to express the object of admiration, referring back to 'the book'. It conveys the sense of being impressed 'by it'.

• أحضرت العلبة الجميلة، وملأتها بحلوى لذيذة وقدمتها لجدي.

  I brought the beautiful box, filled it with delicious sweets, and gave it to my grandfather.

  In this instance, 'بها' is part of the phrase 'ملأتها بها', where it specifies what 'it (the box)' was filled 'with', in this case, delicious sweets.