1. The hospital is for certain people.
2. He has a certain problem.
3. He has a sure problem.
4. He is certain that the boy is diligent.
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Are they all grammatical? The meaning of 'certain' seems to be different. Does #2 mean #3? Is the meaning of 'certain' the same in #2 and #4?
3 is not a standard phrasing; use 2.
In 1 and 2, 'certain' means 'specific' or 'particular.' In 4, 'certain' means 'sure.'