English question, please help?

"sentence to jail" is a PHRASE, commonly used to mean 判處徒刑
"Renata was sentenced to seven years jail . . . ." ( ✓ ) --→ Renata 被判處了 七年徒刑
≡ " . . . . was sentenced to seven years' jail . . . ." ( ✓ )

Q1: Should "year" be plural here?
A: Yes, as it is "seven" years.
In case of ONE year, write: "Renata was sentenced to one year jail." ( ✓ )
　　　　　　　　　　　or, ". . . . . . . . . . . . . . . . . . . . one year's jail." ( ✓ )

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Q2: Shouldn't it be "seven-year jail" ?
A: Yes, you are right to a certain extent. But the sentence has to be put this way:
" Renata was sentenced to a seven-year jail." ( ✓ ) -----→
　　　　　　　　　　　　^^
or, " . . . . was given a seven-year sentence."( ✓ )
　　　　　　　　　^^
However, it's incorrect saying :
" Renata was sentenced to seven-year jail." ( X )