why do we pronounce “f” in enough, but not in although

There are no set rules for ‘gh’ pronunciation.
 
The reason for the large variety of different sounds lies in the history of England and the English language. The ‘gh’ spelling originate from Anglo-Saxon times when words ending in ‘h’ were pronounced with a harder sound, as seen with the Scottish pronunciation of ‘Loch.’
However, the French invaded England in the 13th century, which had a huge impact on the English language and its spelling. To better represent the “thicker’ h sound, the spelling of many h words was changed to ‘gh’.
 
Nowadays, there are 7 main sounds for the ough letter grouping.
Here they are with examples
 
ew – as in “two”

• through, throughout,

 
off – as in “off”

• coughtrough

 
uff – as in “stuff”

• enough, rough, tough

 
oh – as in “no”  

• dough, though, although

 
ow – as in “now”

• bough, drought

 
u – as only the “u” in “stuff.”

• borough, thurough

 
ore – as in “store”

• bought, brought, sought, thought