At some point between the 4th and 7th century AD speakers of what was becoming High German developed a habit of changing the pronunciation of Proto-West Germanic (PWG) words containing a voiceless stop that sounded like p to a fricative that sounded like an f and an affricate that sounded like a pf.
Let’s look at some examples of reconstructed Proto-West Germanic Words and their English and High German descendants the latter of which experienced these sound changes:
PWG *skāp eventually became sheep in English and Schaf in High German. Notice how English retains p while High German replaces it with f.
*opan eventually became open in English and offen in High German.
* plūmā eventually became plum in English and Pflaume in High German. Notice how English retains the p while High German replaces it with pf.
*plōg eventually became plough in English and Pflug in High German.
Becoming aware of these sound changes should lead to better comprehension and vocabulary uptake. I’ll leave you with a list of words that underwent the sound changes p -> f, pf. Let’s see how many of the following you recognise as having cognates with the same or similar meaning in English:
Apfel, helfen, Harfe, hoffen, laufen, pflücken, reif, scharf, Schiff, stampfen, Tropfen.