1. Either 'she' or 'her'. Note, 'were' links the subject "I" with its complement "she":Originally Posted by vladz
This is she. (Subject is Subject Complement)
This is her. (Subject is Object)
Speakers who use the informal structure do so based on a variation of a grammatical rule: the word that directly follows the verb is the object of the verb, and hence use an object pronoun. The problem, however, is that the verb To Be is an exception to the rule: It takes two subject (pro)nouns, one before and one after. I am she. / I am he.
2. Again, 'is', a linking verb, joins the Subject "Who" with it's complement 'younger'. Who = You or I.
EX: I am younger. Who is younger?
3a. to know more than I know. (OK)
3b. to know more than I. (OK; formal)
3c. to know more than me. (OK; informal)
In 3a. "know" is redundant, so in 3b. "know" is omitted, but it's still implied, and so its subject "I" remains; in 3c. "I" is replaced by "me" because the verb is not overt (i.e., spoken/written).
Speakers who use the structure 'than me' do so based on a grammatical rule: If there's a verb after "than", use a subject pronoun; if there isn't a verb after "than", use an object pronoun.
All the best, :D
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