"Has" is never past tense. It's only used in the third person singular in the present tense. "Had" is always past tense.

As mentioned "has" is not used in past tense, but maybe you are mixing up the "present perfect" with "past". My father has a cow. present (My father has a cow now) My father had a cow. past (My father had a cow in the past but not now). My father has had a cow. p.perfect (My father had a cow off and on over the years) The third "has had" form is what they call "present perfect". It can signify a result of an action, past experience, current state, and so on. Any grammar book will explain it in detail - I suggest you go study the "tense" chapters.

'My father had a cow' - talking about the past , so he no longer has a cow (he sold it, it was stolen, it wandered off etc...) 'My father has a cow' - talking about now, he still has a cow, it's in the field :)