Why does gh say f and but sometimes is silent?

If tough is tuff then through should be thruff right?

The "gh" digraph sounds like "f" in a few English words, such as "tough", "laugh", "cough", "rough", and "enough". This is because of a sound change that occurred in the history of the English language.

In Old English, the "gh" digraph represented a voiceless velar fricative, which is a sound similar to the "ch" in the Scottish Gaelic word "loch". However, over time, this sound changed to a voiced velar fricative, and then eventually to the "f" sound that we hear today.

The reason why this sound change occurred is not entirely clear, but it is thought to be related to the Great Vowel Shift, which was a major sound change that affected the English language in the Middle Ages. The Great Vowel Shift caused many of the vowels in English to change their pronunciation, and it is thought that this also caused some of the consonants to change their pronunciation, including the "gh" digraph.

The "gh" digraph still represents the voiceless velar fricative in a few English words, such as "lough" and "ough" (in the Scottish sense of the word). However, in most cases, the "gh" digraph is either silent or pronounced as "f".