English and German are not mutually intelligible because they are different
languages. Nevertheless, they are not that dissimilar.
A speaker of English who would say He swam in the deep water can easily understand the Low German sentence He swamm in dat deepe Water. The High German version is a bit more incongruous to that:
Er schwamm in dem tiefen Wasser. This difference derives from the Second
Consonant Shift which - different to the First or Germanic Consonant Shift that "split off the prehistoric ancestor of the Germanic
languages from the rest of the Indo-European languages" and was first formulated by the philologist Jacob Grimm and later on more specialized by the Danish linguist Karl Verner - occurred in none of the Germanic languages but Old High German and is therefore also called High German Consonant shift. The following table will illustrate the differences it caused:
Old English
Modern English
Old Low German
Modern Low German
Old High German
Modern High German
etan
eat
etan
eten
e an
essen
stan
stone
sten
Steen
stein
Stein
slæpan
sleep
slapan
slapen
slafan
schlafen
scip
ship
scip
Schipp
scif
Schiff
ic
I
ik
ik
ih
ich
The table below will generalize the differences, which were caused by that sound shift:
High German
(shifted)
Low German
(unshifted)
after fricative (i.e. */s/, */f/, */h/)
initially; after consonant other than Germanic fricative; in gemination
medially between vowels; finally
/t/
/ts/
/s/
/t/
/p/
/pf/
/f/
/p/
/k/
, , dialectally
/x/
/k/English and Low German share more language phenomena. As I already pointed out in the introduction, they both have an Ingveonic character.