Why can't English speakers read Beowulf, a poem written in English?

Beowulf is English, but not the English you speak. It was composed in Old English, which grew out of the languages Anglo-Saxon settlers brought to Britain in the 5th century, and which linguists describe as largely incomprehensible to modern English speakers without study.

Two things changed so much that the poem looks foreign. The first is vocabulary: perhaps around 85 percent of Old English words are no longer in use, though the ones that survived are the basic building blocks of Modern English. The second is grammar. Old English nouns, adjectives, pronouns and verbs carried many inflectional endings, and word order was much freer. Over the centuries English traded those endings for a fixed word order.

Here is the odd part: those endings did not vanish because of one big event. After the Norman Conquest of 1066, French became the language of the upper classes, and it is tempting to blame it. But case endings were already wearing away, and the same trend, a little slower, happened in other Germanic languages. One suspect is Old Norse, spoken by Viking settlers: it resembled Old English like a cousin, so speakers roughly understood each other, and in time the endings melted away.

The poem even opens with a word we no longer use. Hwæt is the poet's equivalent of lo or behold, a call to attention like what-ho!