fönstren

In a sentence

• Jag öppnade fönstren för att vädra rummet.

  I opened the windows to air out the room.

  Here 'fönstren' is used to refer to multiple windows in a determinately known context, hence in the definite plural form.

• Vi tittade ut genom fönstren på det vackra landskapet utanför.

  We looked out through the windows at the beautiful scenery outside.

  'Fönstren' refers to the specific windows we were looking through, also in definite plural form as the windows were clearly identified in the context.

• Solen sken in genom fönstren, och fyllde rummet med ljus.

  The sun shone in through the windows, filling the room with light.

  Again, 'fönstren' is in definite plural form, indicating the specific nature of the windows allowing sunlight to enter.