the WUG test:

I think that, through the use of imaginary words, Jean Berko was trying to show that the child does know the rules regarding tense and can apply them "wugging", "wugs" and "zibbing", "zibber" but when it comes to identifying titles of jobs, whereas the adult would know that someone who 'zibbed' was a 'zibber' but the child would say zib man as this is quite an adult term and one they wouldn't recognise as yet. 

I runned- this overgeneralisation is due to the child recognising how to use to "ed" rule but not yet knowing where to use it so they add it to words such as runned as they havent learnt that some words alter when they are pluralised as run tuns into ran.

two mouses- again with this word they child has not learnt to say mice, and adds and s on to the words as it is added to dog(s) and cat(s) etc

my foots- this is the same as above. child had not yet learnt to say feet

he feeled shy- the child has generalised the "ed" to feeled as they have not recognised it should be felt

he satted on the truck-

he drived himself-