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