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I wanted to learn English that is why I asked.
I wanted to learn English that was why I asked.

Do we normally say the 1st sentence for replying immediately to what another person has just said while the 2nd sentence implies a considerable amount of time has passed before a reply was made?

If only the 2nd sentence is correct, is it because of the word wanted (assuming this is really in the past form not the polite form) therefore that was why should be used?

I would be very grateful of your inputs in this matter.
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The punctuation in your examples is wonky. Let's change them to:

I wanted to learn English. That is why I asked.

I wanted to learn English. That was why I asked.

Reasons are usually "eternally true", so it usually hardly matters whether we say "that is why" or "that was why", regardless of when the thing happened. "that's / that is why..." is the more common choice though.
Comments  
Thanks, Mr. Wordy, for your explanation. I really appreciate it. Emotion: smile