is this correct and formal?
"Criminals who have committed heinous crimes should be convicted to a long-term sentence"

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Almost perfect. But sentences should be plural and we don't say convicted to a sentence. So here's my suggestion:
"Criminals who have committed heinous crimes should be convicted and given long-term sentences"

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I'd change the voice to Passive, so it'd be : Heinous crimes committed by criminals should be......

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Or you could say: “Criminals who have committed heinous crimes should be handed long-term sentences”

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Criminals who have committed heinous crimes should be sentenced to long-term imprisonment.

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