o - apostrophe - clock why do English put an apostrophe between o and clock? 8 o'clock.. what does it represent????

tired of sleeping late! Yesterday I couldn't sleep up to 3 o'clock in the morning. I remained in my bed with my eyes wide open! Today I woke up at six o'clock to go to work: I had slept only for three hours! I cannot understand why, since the night before I slept little too and since I work all day long and I'm tired. I feel a bit sleepy now.. but I know that if I go to bed right now this feeling would go away! As usual! My doctor says it's my low blood pressure's fault. Does it happens to someone of you? What do you do when it happens? PS.thank you for you corrections :) and for other answers :) and for reading :)

singular/plural "a surprisingly large proportion of mistake seem to have had their origins in the writer's failure to understand whether the items being discussed in a sentence are singular or plural" shouldn't it be: " a large proportion of mistakes seemS to.." ? isn't "a large proportion" singular? thank you :)

which ONE ? I read this sentence: "Which do you think is the most interesting?" shouldn't it be:"Which do you thing is the more interesting one?". If not..why?! "I THINK LONDON IS THE MOST INTERESTING" or "I think London is the more interesting ONE" ?? Why in some sentence we have to put ONE ? thank you

BETWEEN - AMONG I read this sentence: "I can't make up my mind between Rome, Paris and London". Why "between" is used? There are three of them! I knew that with 2 nouns we have to use BETWEEN and for more than 2 nouns we have to use AMONG. thank you

past tense - present perfect ? I read in an English exercise book this sentence: "..just now he told me that he can't anymore". I knew that if the action has JUST happened the tense to use is the present perfect; so it should be: "he has just told me that he can't anymore". What do you think? :)

H - unvoiced H at the beginning of some words "Silent H : H is not pronounced at the beginning of many words (the text doesn't say which are these "many words"!!!!). Use the article "an" with unvoiced H. Here are some of the most common: hour, honest, honor, heir, herb." Can someone list other words with unvoiced H as a first letter?

to do or doing? When do we use the -ing form and when the basic form? " I like listening to music" " I like to listen " is wrong, why? Is it simply idiomatic?

h - pronounced or not how do we know if the h as a first letter in a word is pronounced or not?? thank you