The original Old English language that was a West Germanic language gradually became Middle English by adding vocabulary and language patterns taken from Norman, or French, language brought into Britain by invaders. It gradually became Early Modern English through the addition of words and pronunciation patterns from many other European languages. Modern English developed gradually as the English language spread through colonization and borrowed words and pronunciation patterns from around the world.
The primary mechanisms that changed English over time were invasions from other countries bringing new languages into the British Isles and the spread of English colonization, which led to the English-speaking population borrowing many words from other languages. Since the advent of the Modern English period in the early 18th century, English has changed primarily in terms of vocabulary and pronunciation, with grammar seeing very few changes over the years.
Old English had a very different grammar than Modern English, but about half of the words were cognates to Modern English words. The Old English grammar system included gender identifiers and case markers for nouns and adjectives, and its sentence order was similar to that of Modern German. During the transition to Middle English, English lost and gained a few letters in the alphabet, and it developed what would become the Modern English sentence structure. English also gained a lot of words from the early French language during the Norman invasion.
The Early Modern Period saw some standardization of spelling through an increase in written works and a shift in vowel pronunciation. Over the years the use of the informal form of you, seen in words such as thou, thy and thine, died out. The biggest changes were the addition of vocabulary from cultures around the world and gradual pronunciation shifts.