Old English originally had three grammatical genders and complex noun cases, just like modern German. It lost them because England was conquered—twice.
If you look at a manuscript from before these invasions, you will likely not recognize it as English at all.
The shift away from this complex Germanic structure started with the Vikings. In the 9th and 10th centuries, Norsemen settled across northern and eastern England in an area known as the Danelaw. Old Norse and Old English were closely related Germanic languages. Their core vocabularies were similar enough that a Norse settler and an Anglo-Saxon farmer could recognize the roots of each other's words, but their grammatical endings—the suffixes that indicated gender and case—were entirely different. To make trade and daily life possible, speakers of both languages began dropping these confusing endings. This contact smoothed away the complex inflectional system, leaving a simplified grammar that relied on word order rather than suffixes to convey meaning.
The second wave came in 1066 with the Norman Conquest. William the Conqueror replaced the English ruling class with French-speaking Norman nobles. For the next 300 years, French and Latin were the languages of government, law, and the church. English was relegated to the peasantry.
Because English was no longer the language of the educated elite, it stopped being formally taught and lost its standardized written form. Unmoored from the conservative influence of scribes and scholars, the spoken language evolved rapidly. The remaining grammatical genders vanished entirely. To compensate for the loss of noun cases, English locked into a strict Subject-Verb-Object word order and began relying heavily on prepositions like "to," "for," and "by."
By the time English reclaimed its status as the national language in the 14th century, it had transformed from a synthetic language that built meaning through word endings into an analytic one that built meaning through word order. It also absorbed about 10,000 French words in the process, leaving modern English with a Germanic foundation, a massive Romance vocabulary, and a grammar that operates like neither.