Could.
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kook jIf the problemwillremains after youdid[have done / do] everything you can ,then so be it.
If the problemwillremains after you did everything you
kook jSo can I say :If the problem remains after you have done everything you can, then so be it. after a person has done something without changing can to could?Yes. When everything is in the present point of view like this, you're basically stating a general principle.