to keep, to maintain

Like Ming said "to maintain" is most appropriate meaning for 유지하다. When it's better (for stylistic purpose) to use "to keep", then the meaning is still same - "to maintain" or "to keep something in a certain state" if you open endic.naver.com, search for 유지하다 and open 예문 더보기 then you can find many examples 모든 국가들과 우호 관계를 유지하다 = to maintain friendly relations with all countries 몸을 유지하는데는 적절한 영양 공급이 필요하다 = The body requires proper nutrition in order to maintain itself "to keep doing something" you can also use 계속+Verb, I guess 비가 오후 내내 계속 내렸다 = The rain kept up all afternoon there are many other verbs for "to keep" for each meaning of the verb you can find all verbs that can mean "to keep" in endic.naver.com if you search for "keep" (지니다, 지키다, 간직하다, 두다, 유지하다) Good luck!

계속: keep doing smt 일을 계속하다: keep on working 이야기를 계속하다: keep on talking 계속 연락해요: keep on touch

I will quote from my grammar source (Using korean - a guide to contemporary usage by Miho Choo) -어 쌓다 ‘repeat excessively’ 아이가 계속 울어 쌓는다. The child is crying non-stop. 정말 잘 먹어 쌓는다. You really do keep eating. -어 대다 ‘do continuously’ 그렇게 떠들어 대니 목이 안 쉴 수가 없지. How can your voice not go hoarse when you babble on like that?

실온을 18로 유지하다 the room temprature is maintained at 18 degree.