Consider the following sentence:
If I knew what the problem ___, I would be able to help you better.
Should the verb be "is" or "was"?
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That sentence is in the irrealis mood, and is commonly known as an "unreal conditional". In irrealis mood broadly, it is always correct to use a verb tense that is one tense in the past from the time they represent.
So in your sentence, the time is present, so it's definitely correct to use the simple past "was" to indicate it is unreal.
Sometimes we can also use the same tense as the time in unreal grammar. This is possible (never required) when the verb refers to a general fact or a long-term state. For example:
If I knew what his job is, I would be able to help you better.
In that context, his job is a long-term state, so "is" is acceptable. In your example sentence, however, "the problem" is probably just a problem now, not a stable, long-term state, so only "was" is correct, not "is".