Rubah basically has it correct. West Germanic tribal languages + Latin = Anglo Saxon + 500 years = Old English + French = Middle English + 500 years = Modern English.

Basically, West Germanic tribal languages took words from Latin to form an Anglo-Saxon language. They invaded Britain and there the language consolidated within itself to become Old English (which 'Beowulf' is written in). The Normans conquered Britain in 1066 and brought their French (called Anglo-Norman). Over the years it combined with Old English to create Middle English ('Sir Gawain and the Green Knight', Chaucer). In the 15th and 16th century, slight changes due to emigration from the Black Death resulted in a switch in vowel pronunciation which brought Early Modern English (Shakespeare). That underwent little changes until we get what we have today.