(1) Mary's hair is longer than mine.
=> mine = my hair.
You are comparing “Mary’s hair” with “my hair”
He is shorter than I (am).
“He” is a subject, comparing to “I” – a subject. (than is used as a conjunction)
I am taller than him
=> A colloquial usage (informal), assuming “than” to be used as a preposition. 200 years of elaborate reasoning have led to these present-day inconsistent conclusions: than whom is standard but clumsy.
(2) The lost girl was frightened when she heard strange noises in the woods.
The girl was frightened by strange noises in the woods when she heard them.
Strange noises frightened her.
= She was frightened by strange noises.
She is frightening older people.
* meaning => Older people are being frightened by her.
See http://www.northeastern.edu/toolkit/messaging/style13.html around passive/active voice.
(3) Do you recognize the girl who is wearing a red shirt?
Do you recognize the girl who wears a red shirt?
Do you recognize the girl wearing a red shirt?
“Do you recognize the girl wears a red shirt?” is wrong with two verbs in a clause. You need a relative pronoun "who" to connect the two clauses or a present participle to modify the noun "girl".
See http://www.englishpage.com/verbpage/presentcontinuous.html for the usage of present continuous tense.
(a.) Use the Present Continuous with Normal Verbs to express the idea that something is happening now, at this very moment. It can also be used to show that something is not happening now.
Who can tell the color of the shoes (that) Amy is wearing now?
I am looking for the person who is wearing the largest pink button.
See http://www.diva-girl-parties-and-stuff.com/pink-pajama-party.html. "is wearing” 不一定有正在穿衣服的意思, 可以指我在此刻穿的衣服。
(b) Sometimes, speakers use the Present Continuous to indicate that something will or will not happen in the near future.
I am wearing a pink skirt to Jamie’s house tonight.
** 版權所有 - Elisa