Hi Frazer,

This is an interesting one! Of course you're right - it should be 'would have', not 'would of'.

It must have (not 'must of') started in speech, since 'would have' and 'would of' sound very similar in natural speech. However, nowadays it's often written as 'would of' too.

One website I looked at earlier said that the English language is always evolving, and maybe if an error occurs often enough and becomes the norm, then it shouldn't be considered an error any more.

What do you think?

Grammarman