Old English is also markedly different from Modern English in that nouns, verbs, adjectives and articles were inflected for case, number and gender. This meant that Old English had a much freer word order than Modern English (shared by an ancient language still read today - Latin), which is locked into the strict Subject-Verb-Object pattern. The inflectional system was a characteristic inherited from proto-Germanic, from proto-Indo-European and has not survived in Modern English, with some exceptions, notably in pronouns.
This meant there were far fewer prepositions in Old English than in Modern English. Instead Old English words were synthesised with suffixes.
In addition, many consonants of Old English have evolved into other, or different consonants in Modern English, many vowels moved from long to short (Great Vowel Shift) and there were some sounds such as a glottal stop (/./) that are heard in only few dialects of English today.
Old English was also influenced in domestic vocabulary and pronouns by Old Norse (a North Germanic language), due to Viking invasions of England (or Aengleland) several centuries after the Aenglish settlement of Britain (Pridain).
A useful text for further reading:
Barber, Charles (1993) The English Language A Historical Introduction Cambridge: The Cambridge University Press