<p>I believe that you’re talking about the present perfect vs the past simple. Here are 3 links: [The</a> Present Perfect Tense and the Simple Past Tense](<a href=“http://web2.uvcs.uvic.ca/elc/studyzone/410/grammar/ppvpast.htm]The”>Present Perfect and Simple Past | Continuing Studies at UVic)
[Simple</a> Past vs. Present Perfect Simple](<a href=“http://www.ego4u.com/en/cram-up/grammar/simpas-preper]Simple”>Simple Past vs. Present Perfect Simple - English Grammar)
[Past</a> Simple or Present Perfect 1](<a href=“http://www.better-english.com/grammar/pspp1.htm]Past”>English Tenses - Learn English for Free)</p>
<p>In regards to post #4, I definitely agree with the meaning of “I have studied math for the past four years”, although I think it’s better to say “I have been studying math for the past four years”. However, the last sentence sounds wrong to me. If I want to say that in the past I studied math for four years, but am no longer doing so, I would simply say “I studied math for the past four years”. Honestly, even that doesn’t sound quite right. I can’t provide a reason as to why though. It’s called the past perfect, in case you want to look it up.</p>
<p>Okay, I’ve done more thinking. I think that the last sentence bothers me because the word “past” lets the listener/reader know that you’re already talking about the past. Therefore, the word “had” isn’t necessary…just my 2 cents</p>