There is certainly a difference in meaning, which embodies the difference in meaning of the two tenses: "rained" is simple past, and implies a completed action entirely in the past, while "has rained" is present perfect, and implies a completed action up to and inclusive of the present moment. The differing implications to me of these two tenses is that "it rained a lot" = there will be no more rain, while "it has rained a lot" = there might be more rain.
You can read up on these various tenses to clarify for yourself the differing nuances. Here are a couple of decent ones: http://www.englishpage.com/verbpage/presentperfect.html, http://en.wikipedia.org/wiki/Imperfective_aspect. My understanding is that verb tenses are classifications by linguists that span languages, so in learning about the present perfect tense, you are clarifying its function for all languages you know or might learn.
Also, Spanish has forms that much more directly translate to It rained. llovió mucho este verano and llovía mucho este verano. These carry the same different sorts of meanings as English It has rained a lot and It rained a lot. While you could translate any one of the three Spanish phrases as either of the two English phrases, learning the nuances of how the various verb conjugations translate to English is a good idea.
I disagree, Swingophelia explained it well. The present perfect goes up until the present moment. The past (preterite) has been completed (in the past.)
This is an important difference. If by "colloquial," you mean people who speak carelessly, with little interest in accuracy, you may be correct. But I don't see that as "standard" English. I think of "standard English" as the English taught in school, and used by writers, news reporters, and professional people.
I make the distinction between the two by putting myself in and out of the summer season. If it is July, then you would say "It has rained a lot this summer". If it is December and you're referring back to the summer that recently passed, then you would say "It rained a lot this summer".
I was told by a lovely woman trying to tutor dense Anglos in the preterite tense that using the present perfect tense would be OK, and would only require memorizing the conjugation of haber, how to form the present participle, and a few irregular verbs, instead of all of the conjugations of irregular verbs needed for the preterite. However, she also said it would sound stilted and odd (although understood) unless it were used much as we would use it in English. The two overlap, but aren't exactly the same. 02/12/14
Does anyone know a good website that would help me review the regular conjugations for different tenses?
I'm picking up the translations in the context of the particular lessons but want to make sure i understand the normal patterns for present, past, present perfect, etc.
In my opinion both "It rained" and "it has rained" should be accepted. In many other DL exercises Spanish present perfect is translated to both present perfect and simple past in English. If there is no context and in this example there is no context, then there is no way one can distuinguish, which translation is better.
The Spanish present perfect is used in the same way as the English present perfect in the vast majority of situations, so that's how you should translate it without a context, unless it gives an unintelligible result. Most of the time replacing present perfect with simple past is just sloppiness, which should be avoided when you are trying to lay down the foundations of your grammar.
Understanding the difference between these tenses will allow you to understand the exact meaning of the sentences that use these tenses, rather than some loose approximation of the meaning.