The use of the simple present tense indicates that a statement applies now.
The use of a modifier such as always or never indicates that the statement it modifies universally applies (always) or doesn't ever apply (never). It is usually used to indicate that the statement relates to a habitual or reoccurring state or action.
When different tenses are used with the modifiers always and never, subtly different meanings are implied.
- The simple past tense indicates that the statement applied at all times in the past, or that it universally applies and the past is the most relevant reference point to your statement. It can be used to imply that the statement no longer applies, especially if used with before. E.g. It always did that before. = at all occasions in the past it did something (that), but on the most recent occasion(s) it has stopped doing "that". Universal application can be inferred from context, it is usually used with never and often means that the point at which it could have happened has passed. E.g. I never climbed Everest. = At no point in the past did I climb Everest, and implies that the point at which I could have climbed Everest has passed, so I will also not climb Everest in the future.
- The simple present indicates that it applies at all times in the past
and present. It can imply future continuation, but does not
necessitate it. E.g. It always does that. = it currently does
"that" and did "that" at all times in the past. It implies that it
will probably continue to do "that", but leaves the possibility that
it may stop doing "that".
- The simple future indicates that it applies at all points in the
future, and it usually indicates that it applies at all times past,
present and future (but it may not necessarily). E.g. I will never
climb Everest = I have not yet climbed Everest, and I will not climb
it in the future. If you include the adverb "again" with never, it
means that it occurred previously, but will not occur in future. E.g.
I will never climb Everest again = I have climbed Everest, but I will not climb it in the future.
So in your example, I never know implies that the situation has come up before, repeatedly, and Harry has not previously known, and still doesn't know.