The basic idea is that tense go together. In the conditionals, you can see it:
Zero and first- both use the present tense in the if-clause and the present in the conditional, although the first uses the simple present while the first uses the present modal 'will' mostly.
The second uses the past in the if- clause and the past tense modal in the condirtional clause.
The third uses the past perfect and the conditional perfect.