What grammatical changes have taken place between Old English and Modern English?

Answer 1

Here's an example:
Humour, Flavour, Colour - Old english; in modern english (US English, rather), the u is dropped.

Theatre, Centre - Old english; modern english the re is inverted

Plus, we can't pronounce 'aluminium' to save our lives so we dropped the i

Answer 2

Oh goodness, a whole lot. English lost gender and case, for instance. The word THOU is no longer use. Also the conjugation simplified. We say "you do" and "he does" instead of "he doth" and "thou dost., for example.

Answer 3

Sounds dlifferent, spelled the same for the most part.