The sentence is referring to the past so it should be:This morning I watched TV before I drove here.
What grammatical error is the sentence? This morning I watch TV before I drove here
Since there are two actions and both are in the past so the one which happened first should be in the past perfect tense.
So, it is correct to say (This morning, I had watched TV before I drove here.)
So, it is correct to say (This morning, I had watched TV before I drove here.)