1. Which common three letter word can be prefixed to (placed before) each of SIGN, DONE, DUCT, FOUND, FIRM, TRACT, DENSE, CAVE, JUNCTION, VERSE, and VEX to generate eleven new words?


2. Which common four letter word can be prefixed to each of LOCK, STRONG, WATER, WIND and MASTER to generate five new words?


3. Which common four letter word can be prefixed to each of FALL, FOWL, MARK, SHED, SIDE and WAYS to generate six new words?


4. Which common four letter word can be prefixed to each of ARM, CAST, PLAY, SHADOW, TELL, WARN to generate six new words?


5. Which common four letter word can be prefixed to each of TRAP, PLACE, BUG, MAN, FLY to generate four new words?


6. Which common four letter word can be prefixed to each of SCRIPT, MAN, MARK, AGE to generate four new words?


7. Which common five letter word can be prefixed to each of BIRD, CLUB, FALL, JAR , and SCHOOL generate five new words?


8. Which common five letter word can be prefixed to each of RUNNER, AGE, LINE, RANK, MAN to generate five new words?

3. WATER
The number of letters for the clue #3 should be five, not four.
Waterfall, waterfowl, watermark, watershed, waterside, and waterways are words that demonstrate that the five-letter WATER can prefix each FALL, FOWL, MARK, SHED, SIDE and WAYS.  I did not find a four-letter solution.

7. NIGHT
Night school is not an open compound word, thus, NIGHT does not really prefix SCHOOL. (Though, it is used as a closed compound word in the English-language manga [comic book/illustrated novel] written and illustrated by Svetlana Chmakova where supernatural beings go to school. But I think this does not really qualify it as a "valid" word.)