My first question is: generally, how can we use italic form? Does it matter to how long the quote is?
Secondly, when we open a pare of bracket(parenthesis), should it be sticked to the last word?
Both are very good questions.
First, we use italic text for two main reasons: to place emphasis, and to identify a particular kind of document like a book or journal. We may also use italics as I just did with Majid's question, as a way of quoting without using quotation marks.
In the typewriter age, italics were for printers, not typists. The best we could do was to underline text that we wanted to see in italics. (With the IBM Selectric, we got type balls with italic letters—they were fun, but clumsy.) Now, however, we can use (and abuse) italics as much as we like.
So we can write:
This test is much faster
—and the italics help distinguish "this" test from others. Or we can write:
This test is much faster
—and italics show that the test is very, very fast.
We should use such emphasis very sparingly, however. In the examples I've given, "this" and "much" should be enough emphasis.
We also use italics for the titles of books, magazines, scholarly journals, and major works of art:
Anatomy of Criticism by Northrop Frye
Journal of Quaternary Studies
The Tale of Genji
For articles within books and journals, and short stories and poems, we use quotation marks:
"Blake After Two Centuries," by Northrop Frye
"Will Iran be Next?" by James Fallows
"A Rose for Emily," by William Faulkner
"Little Gidding," by T. S. Eliot
Italic text draws attention to itself (and creates emphasis) because it's rare. We're used to roman text, like most of the text in this post. I recommend avoiding use of italics for more than one or two paragraphs. Otherwise your readers will grow tired. (By the way, if you must emphasize a word or phrase within a long passage of italics, just use roman text.)
As for parentheses, we use these when we must interrupt a sentence or paragraph with some kind of additional information—as I did in the last paragraph. Again, we use them sparingly because they make the reader stop, instead of moving along with the help of commas, semicolons, and dashes.
When using parentheses within a sentence (as I'm doing here), note that any punctuation you would have used will go after the close of the parenthesis. In my example I needed a comma after "sentence." Instead it went after the parenthetical material. We don't put any extra space between a parenthesis and the word or letter next to it.
(If a whole sentence or more is in parentheses, then it takes a capital letter at the beginning and a period, question mark, or exclamation mark at the end. As in this example, a whole paragraph can be in parentheses. If you must do this, however, perhaps you should think hard about how to blend this interruption into your main text!)
Brackets are slightly different. We use them for parenthetical material within parenthetical material:
(He was a pupil of Socrates [469?-399 BCE] and a well-known political figure.)
(Now I need parentheses again to interrupt myself about two small points: The question mark after the first date means we don't know for sure when Socrates was born, and "BCE" stands for "Before the start of the Common Era." "BC" stands for "Before Christ," but many non-Christians also use the same calendar. So it is preferable to use "BCE.")
We can also use brackets when we are inserting our own comments into a quotation, and we want to make it clear that our original source did not use our words. Suppose, for example, our source said:
"I think he is seriously mistaken" and our readers won't know who "he" is. We can write:
"I think [John Smith] is seriously mistaken."
Thanks for your questions, Majid!