What is a preposition?

[Sibylle]

Test No. incompl/elem-13 "Grammar Questions", question 3

Which of these is a preposition? It is ..........

(a) thorough
(b) trough
(c) through
(d) throng

Test No. incompl/elem-13 "Grammar Questions", answer 3

Which of these is a preposition? It is through.

Correct answer: (c) through
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What is a preposition?

Sibylle

[Torsten]

A preposition is a part of speech (a word type) that serves to express the relationship between two words, for example:

with him
to you
through the window
between you and me
for us

etc.
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Test Of English for International Communication
TOEIC Preparation & TOEIC Vocabulary

[Alan]

Literally it's a small word that goes before (that's a preposition) a noun and usually gives some some idea of direction in time or place. So we have in the room/on the table/after the dance/before Tuesday and so on.
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English as a Second Language
You can read my ESL story Conditionals

[Livia Mackie]

But I thought (very similar words...Oh! my gosh!) that through is an adjective or adverb. What is about "though"? What is that?