The words threw, through, and thru are homophones: they sound alike, but threw and through have different meanings, and thru is generally used only in certain informal contexts.
Threw is the simple past tense of the verb throw, which usually means to cause something to move through the air. (“Buddy threw the ball.”)
Through can function as an adjective, adverb, or preposition, each with a variety of meanings.
Through often suggests a passage—from start to finish, or from point A to point B. (“Buddy walked through the museum.”) Through can also mean finished, over, or completed. (“Buddy is through with school.”) As a preposition, through means by, using, or as a result of. (“Buddy found out about the job through an ad on Craigslist.”)
Thru is an informal spelling of through–usually more appropriate in a text message or tweet than in a formal essay or report.
Lena threw me a kiss as she ran out the door.
“He had read somewhere a story about a woman who threw the handset of her phone at an attacker, and killed him. Needless to say, there had been some luck involved in that eventuality.”
(Gish Jen, “Birthmates.” Ploughshares, 1995)
“Chris Bostic . . . stepped up as calmly as if he were ringing a doorbell and threw a perfect spiral through the target.”
(Mike Lupica, Million-Dollar Throw. Penguin, 2009)
“Mum and I have already stepped in through the small black iron door, which the wind closed like an unseen butler, and currents are pulling us up the garden, around by the wall.”
(David Mitchell, Slade House. Sceptre, 2015)
“I went to the kitchen. Nancy was through. The dishes were put away and the fire was out. Nancy was sitting in a chair, close to the cold stove.”
(William Faulkner, “That Evening Sun Go Down.” The American Mercury, 1931)
I’ll call you when I’m through [or, in a text message, thru] writing my essay.
“The chain’s ‘drive-thru‘ service allows customers to pre-select a collection time and collect shopping from a refrigerated van parked in a designated area of a store’s car park without having to leave their cars.”
(Reuters, “Sainsbury’s to Double Number of Click & Collect Sites.” The New York Times, May 3, 2016)
The word through appears in numerous English idioms. Here are just a few of the more common ones.
The expression through and through means completely, thoroughly, or throughout.
“The storm had broken over them toward midnight, the wild westerly gale swooping at them down the shoulder of the mountains like a wild thing that wanted to destroy them; whipping the waters of the loch into racing white-caps, bringing with it the bitter, hissing rain to drench them through and through.”
(Rosemary Sutcliff, The Eagle of the Ninth. Oxford University Press, 1954)
The phrasal verb go through (with) means to experience, examine carefully, perform, use up, or complete.
“A gush of panic rose in me like gorge. How, I asked myself, how could I stay here? How could I have thought I could stay here, all alone? Well, too late now; I would have to go through with it. This is what I told myself, I murmured it aloud: I shall have to go through with it, now.”
(John Banville, Eclipse. Knopf, 2001)
The expression go through the roof means to get very angry or to rise to a high level.
“In the peak season between July and September prices go through the roof: a hotel room costs twice as much in these months as in April or October.”
(Madeira and Porto Santo Marco Polo Guide, 2012)