It is generally considered that English and all forms of German and Dutch
descended from a common ancestor
, which we may call proto-West Germanic
. The distinguishing features of High German
(the kind you learn in school), principally the Second Consonant Shift
, are innovations in High German. Dutch and Plattdeutsch are collectively known as Low German. So there is not a single ancestor that English and Plattdeutsch share but that High German doesn't. English is from the dialects of the Angles and Saxons, coastal peoples.
However, there are phonetic grounds for suggesting that the High/Low distinction did exist much earlier in Germanic, so the above might not be right. The first division might have been into Low West Germanic and High, with Anglo-Saxon being a branch of Low.
Low German has maken where High German has machen, likewise wat for was, and Appel for Apfel. The lines dividing the areas where one or the other word is used are called isoglosses, and in this particular case form an interesting pattern called the Rhenish fan. They are spread quite far apart along the River Rhine, between north and south, so at many points the local dialect sounds partly Dutch and partly schoolbook German. However, further east the isoglosses converge and there is a sharper division between Low and High dialects.