past simple - "I didn't know Ed was Welsh" or "I didn't know Ed is Welsh"?

I came across these two sentences and asked myself whether I could always use both versions:

I didn't know Ed was Welsh.
Did you know that Cliff's wife is Canadian?

Both relates to someone's nationality. I assume both people still live. So could I say I didn't know Ed is Welsh? Or as I know now - do I have to use the simple past (was) here?

Answer

Mixing present and past happens in some occassions. You can use present simple instead of a past tense when something is permanently true as in

He taught me that knowledge is power.

He told me the Earth goes round the sun.

He said he loves icecream.

In your example also if the guy said that he was Welsh, he's still Welsh I suppose at least till next time that I see him to make sure he's still Welsh or has taken refuge in another country to be the national of that country.

You can use both sentences then:

I didn't know Ed was Welsh.

I didn't know Ed is Welsh.