Main spellings (~95%)
⟨f⟩ ~75%
The most common spelling by far.
⟨ff⟩ ~10%
Doubled after short vowels in the middle or end of words.
⟨ph⟩ ~10%
Common in words from Greek.
Greek roots: ⟨ph⟩ appears in words with Greek origins — phone (sound), photo (light), graph (write), phrase (speak). If a word looks "scientific," try ⟨ph⟩.
Unusual spellings (~5%)
⟨gh⟩
A handful of common words spell /f/ with ⟨gh⟩. These are all from the same Old English sound that changed over time.
Memorize these: The ⟨gh⟩ → /f/ spelling only occurs in a few words: enough, laugh, cough, rough, tough (and their derivatives like laughter, toughen). In most words, ⟨gh⟩ is silent (night, thought, through).