The combination gh is never pronounced. This happens in the past and past participle forms of some verbs, such as for example: thought (from think), and in some other words like weight or light. As you can hear if you click on the words above, we only pronounce the final t, and never the combination gh.
Words starting with gh-.
They aren’t very frequent and they have sometimes been adopted from other languages. In these cases, the gh- combination is pronounced /g/, as in gap. We have for example ghetto.
Words containing -gh- in the middle.
When a word contains the letters gh in the middle we have different possibilities for its pronunciation:
We pronounce /f/ in derivatives of the words in 1A: laughing or coughed.
We keep it silent in derivatives of the words in 1B: higher, sighed.