("need" is used as a modal verb only in present tense questions and negations)
*Ruth needs to see the doctor today.
*Does Ruth need to see the doctor today?
*Need Ruth see the doctor today?
*Ruth doesnt need to see the doctor today.
*Ruth neednt see the doctor today.
*They didnt need to return the books to the library today ( = it wasnt necessary and we dont know if they did or not)
*They neednt have returned the books to the library today. ( = it wasnt necessary but they did nonetheless.)