German is a Germanic language. So is English. So, why do English speakers find it so blasted hard to learn German? Two reasons: vocabulary and grammar. Yes, German also has a few sounds that English doesn't have, but these are as nothing compared to the wildly different vocabulary and the mildly different grammar patterns that are just different enough to cause constant conflict with our own.
Who is to blame for this unfortunate estrangement of siblings? A certain Englishman and a certain Frenchman. The Englishman was the last king before William, known as Edward the Confessor. Edward's conception of religion led him to believe that he would be more blessed by God if he remained chaste throughout his life. Maybe he felt personally blessed, but his failure to father an heir proved a curse to England. Upon his death, a vicious war of succession broke out. It was won by William, Duke of Normandy, who had a real, though weak, claim to the throne. As William was French, he brought in a great many Frenchmen to help him rule his new kingdom. Over the course of the next few centuries, the hardy Anglo-Saxon language came under the strong influence of the newly prestigious Norman dialect of French, the language of the nobility. This affected English phonology, morphology and, most especially, its lexicon.
The phonological changes in English were not severe enough to cause a great deal of difficulty with relation to German. The main effect was that certain sounds that had previously been allophones (variants conditioned by the sounds around them) became distinct phonemes. Thus, f and v used to be variants of a single phoneme (a sound considered "the same" by native speakers of a language). The f sound was used at the beginning or end of a word, or within a word before a voiceless consonant. The v sound was used inside a word between vowels or before a voiced consonant. The same conditions applied to s and z, voiced th as in "the" and voiceless th as in "thin." In French, though f and v, s and z were distinctive phonemes, that is, these sounds could distinguish one word from another. This came to be the case in English as well. This is why, in Old English, in the word heofon the letter "f" was pronounced like the "v" of the modern English equivalent, heaven.
Changes in morphology, that is, word structure, were much more noticeable and did create serious difficulties for English speakers learning German. Noun plurals, in particular, became simplified. The consonant s, with phonologically conditioned variant pronunciations, became the almost universal marker of plurality. It had served as the plural in some cases in some declensions in Old English, but the fact that this was apparently the most common plural ending in Norman French no doubt helped make it the standard in English. Also, the highly simplified French case system helped English lose all morphological noun cases except the genitive (the possessive "apostrophe s" of Modern English). German, on the other hand, has retained its complex case system up to the present day. This creates considerable difficulties for English speakers learning German.
The largest barrier to the easy learning of German by English speakers is, in my estimation, the vocabulary. The core vocabulary of the two languages (i.e., about the 200 most frequent words) is largely cognate (i.e., derived from the same Proto-Germanic root and recognizably similar). Thus help/helfen (verb), go/gehen, eat/essen, man/Mann, etc. But once you get out of this core vocabulary, there are huge differences. This is due to the absorption of large quantities of French vocabulary (ultimately from Latin) by English, as well as a considerable volume of Greek, in contrast to the decision of German speakers to coin new words by joining existing German roots rather than borrowing Latin and Greek terms willy-nilly as English has long done. Thus, for example, in linguistic writing, where English says sentence construction (two Latin words), German says Satzbau (one word with two German roots). In the realm of religion, English uses baptism (from Greek), while German has Taufe. English history (Greek), German Geschichte. English society (Latin), German Gesellschaft. And so on and so forth. The result is that English speakers learning French, Spanish or Italian will immediately recognize thousands of cognates, while in German they will only find a few hundred, and most of the technical terms needed for a particular field of learning will be completely different. So English-speaking graduate students face a daunting amount of vocabulary learning to make even modest progress in German, whereas once they have learned a couple hundred words of non-cognate core vocabulary in a Romance language, their path is clear because of the overwhelming mass of cognates in technical vocabulary.
In a separate post, I will deal with syntactic conflicts between English and German that cannot be blamed on William.