Past tense in the main clause triggers imperfect subjunctive? Why is this in present subjunctive?

Hola Allison,

There is a paragraph in the lesson that is talking about the possible use of present subjunctive when the main verb is in the present perfect (a past tense). But we also have this lesson where you can see that it can be used with both the present and the imperfect subjunctive. 

I hope this clarifies it.

Saludos,

Inma