"Weak grade" is actually just a fancy way of saying "weak".
In English, a weak past participle is one which is created by simply adding something to denote past tense to a verb stem. A good example is "talk", for which the past participle is "talked". There used to be more choice than there is today about what might end it. Sometimes the final consonant is doubled, sometimes not. The use of "t" instead of "ed" or "xed" is the only other ending in use I can think of at the moment, e.g. "spilt milk" rather than "spilled milk".
An example of a strong past participle is "spoke", the past participle of "speak". The form of the word changes, not just the ending, so to speak.
This distinction is virtually meaningless today, except as an arcane subspeciality of grammar. It was much more important in old English.
However, if you are a furrener learning English you would have to learn the weak and strong past participles by rote because there are no reliable rules I can discern which would allow you to determine which is which.
If you've ever listened to a child learning to talk and applying rules to derive words they've never heard or can't remember, you will have had him/her saying things like "speaked", because they understand, in general terms, the rules for deriving weak past participles. The funny thing is, they often seem to "know" that it's wrong, even though they don't know why, or what the correct word to substitute for their made-up PP is.
I went looking for my copy of Fowler to confirm the above, but it's still packed away. Therefore, EO&E.
HTH!
