The changes we make to tenses and vocabulary must make sense to the listener:
It's always correct to change the tense, but it's often not necessary. Similarly in these sentences:
We don't have to change a present tense if the situation is still true (also when it's a general truth):
But when we want to say that the original sentence was not true, we should change the tense:
Other words can also change according to the situation:
Expressions of place can change in a similar way:
The first conditional refers to both the present and the future, so it can stay the same or it can change:
But the second and third conditionals never change:
As we can see, the difference between the real and unreal conditionals may disappear - probably because it's more important to know what time the sentence refers to.
Similarly, the past tenses after wish, would rather and it's time also remain unchanged:
The past simple and progressive tenses refer to the time of speaking. We use the past perfect tenses to refer to something before that time.