In English (as also in Spanish as we will see), the tense to be used depends on the tense of the introductory clause.

He says (that) if you scratch a senator from Texas you will find a cowboy. He has said (that) if you scratch a senator from Texas you will find a cowboy. He will say (that) if you scratch a senator from Texas you will find a cowboy. He said (that) if you scratched a senator from Texas you would find a cowboy. He had said (that) if you scratched a senator from Texas you would find a cowboy.

So far, so good. How does it really function?

1) if the introductory clause is in present tense, present perfect, future tense or conditional then the forms of the quoted sentence are not changed.

2) if the introductory clause is in simple past tense or past perfect the forms of the quoted sentence are changed as follows.

direct speech

reported speech

present tense ==> (he said) "I like living in London" present continuous ==> (He said) "It's raining cats and dogs" present perfect ==> "I have waited here already for two hours" present perfect continuous ==> "I have been waiting here for three minutes" simple past tense ==> "She went to China" past continuous ==> "She was running wild"   past perfect ==> "He had gone home before you came"   past perfect continuous ==> "He had been waiting for two hours before he left"   future "I will be a movie star"

past tense He said that he liked living in London past continuous He said that it was raining cats and dogs. past perfect He said that he had waited there already for two hours. past perfect continuous He said that he had been waiting there for three minutes. past perfect He said that she had gone to China. past perfect continuous He said that she had been running wild.   past perfect He said that he had gone home before you had come.   past perfect continuous He said that he had been waiting for two hours before he had left.   conditional He said that he would be a movie star.

Then of course, there are some exceptions to this rule.

a) If the main clause something general, common knowledge or eternal truth the verb can be unchanged (in present tense) even if the introductory clause is in past tense.
b) in spoken language the simple past tense can be unchanged even though it's against the rule.

He said "She went to China" ==> He said (that) she went to China.

(The conjunction that is very often left out.)

Now let's have a look at the Spanish system, that is in itself not so very different from the English one.