At some point between the 4th and 6th century AD speakers of what was becoming High German developed a habit of changing the pronunciation of Proto-West Germanic (PWG) words containing a voiced stop that sounded like t to two different affricates that sounded like a sharp s represented in the written language by “ss”, “s” and “ß” and ts represented by “z”.
Let’s look at some examples of reconstructed Proto-West Germanic Words and their English and High German descendants, the latter of which experienced said sound changes:
PWG *etan eventually became eat in English and essen in High German. Notice how English retains t while High German replaces it with ss.
* ūt eventually became out in English and aus in High German.
*tehan eventually became ten in English and zehn in High German. Notice how English retains t while High German replaces it with z.
*tungā eventually became tongue in English and Zunge in High German.
Becoming aware of these sound changes should lead to better comprehension and vocabulary uptake. Next time you come across a preposition, a word for a number, a body part or an action in German that contains ss or z, see if you end up with something that approximates the English word for the same meaning! You’ll be amazed how effective this process of “reverse-engineering” the language can be.