The Present Perfect will be easier to understand if you understand it first in English. Try these websites:
Learnamericanenglishonline.com
or
Grammar Websites
StudySpanish.com- This website contains descriptions of grammar and practice quizzes. Best for beginner students.
SpanishDict.com- This website contains descriptions of grammar and also includes a dictionary, verb conjugator, and practice quizzes.
Spanish.About.com- This website contains descriptions of grammar and usually has good examples.
IPFW- University of Indiana/Purdue University website that has detailed grammar explanations on a low tech site. Great for advanced students and students that like the full story.
WordReference.com Verb Conjugator-Need to know the forms of any verbs in Spanish? Type the verb in here. (Note this will give you the conjugations for many tenses, make sure you are looking at the right one)
Practice Activities
Practice Drill- From StudySpanish.com
Practice Quiz- From StudySpanish.com
Practice Test- From StudySpanish.com
Practice Quiz- From SpanishDict.com
Practice Activity- From Colby.edu
Practice Activity- From Conjuguemos.com