Prescriptive grammars, although useful for teaching any second language, need to be used with care. Prescriptive grammars make broad statements about how a language is used, giving shortcuts expressing general rules that can be applied more widely than the few examples you can cram into a textbook. But the shortcomings of prescriptive grammars are
1) as languages change, the statements made can get badly out of date
2) the recommendations can be applied in a context that is wider than the linguistic evidence supports
3) they can be plain wrong!
Don't blame Gerard. It was I who woke you from a sound sleep.
Don't get mad at me! I didn't pull your ponytail! It was he.
Remember the amazing guitarist I met? This is she.
As a native speaker with a degree in linguistics, I disagree with all three of the above examples... I would say 'It was me who woke you,' 'It was him,' and 'This is her'. Yes, there are grammatical arguments for the answers given, but they are all misleading — traditional grammarians tend to look back to the stricter formal grammar of Latin and try to apply it to modern English. Latin grammar was never applicable to English. The resulting sentences would all be stilted and unnatural to a modern native speaker.
The simple answer is that, if a sentence sounds stilted, it is wrong. If you are not sufficiently proficient in English to make this decision yourself, refer to a native speaker, or at least a more proficient speaker, one who you trust to be able to differentiate between sentences that are well-formed and those that are not.
Linguists today work descriptively, describing how a language is used, not prescriptively, prescribing how a language should be used. Language teachers are caught in a bind — language teaching by its very nature is essentially prescriptive. A safer way to impart language is to say "You tend to hear A" rather than "You should say A".
As a case in point, in a paragraph above I have said 'one who you trust'. A prescriptive grammarian would tend to recommend 'one whom you trust', as 'whom' is in the objective relation to 'trust', but modern English usage is fast losing this distinction! Instead of saying "You should use 'whom'", or "You should use 'who'", I would say "Most of the time, you will find the word 'who' in this construction." Even better, if I had some statistics to support my contention (which I don't!), I would say "75% of the time you would find 'who' in this construction." Actual usage will always trump theory.