'to the head teacher's office'

One way to obtain the Welsh equivalent of this type of English phrase is to:

• put the English phrase the long way round:
  to the head teacher's office → to the office of the head teacher
• get rid of the first 'the' and the 'of' in the phrase:
  to the office of the head teacher
  to office the head teacher → i swyddfa'r pennaeth

Remember that the preposition i causes a soft mutation. Also y and r will cause a soft mutation if they are followed by a feminine singular noun, e.g.

to the middle of the bridge → i ganol y bont