It is very important to use VERB TENSES correctly and here you have some EXERCISES to practice.

https://clasejoseangel.files.wordpress.com/2010/04/mixed-tense-revision-4.pdf

https://cristinasantavictoria.jimdo.com/recursos-didácticos-por-niveles/2º-bachillerato/ejercicios/H

Here you have the link of a summary of VERB TENSES:

https://docs.google.com/viewer?a=v&pid=sites&srcid=ZGVmYXVsdGRvbWFpbnxkcHRvaWRpb21hc2luZmFudGVzfGd4OjRjYTc0MmZmNmJmMzE2Ng