The following are errors in English:

* I should of known better.

* They could of beaten us.

* He must of left by now.

* an asterisk in front of a sentence denotes an ungrammatical sentence.

In good, grammatical English we say instead:

I should have known better.

They could have beaten us.

He must have left by now.

All these examples use the correct past forms of the modal verbs‏‎, should, could and must.

Pronunciation

If we transcribe should have into the IPA it will look like this:

/ ʃəd hæv /

However, this is very precise, clearly spoken, English. If you listen to the language spoken naturally by a native British or American English speaker the pronunciation is very different. This is due to the way in which speakers will often slur the words together (known as elision).

Thus the speaker says instead:

/ ʃədəv /

Also, when we write informal English, we’ll often put:

should’ve

could’ve

must’ve

Thus because of the pronunciation and the informal written version, some illiterate people have assumed that the correct English is should of (could of, must of etc) because this is what it sounds like.

Note that pronouncing should have as should of is fine however it must always be should have when written.