I think the d / t issue goes even beyond that though, right? For instance, the past tense of some verbs are spelled with a final t in UK English and an ed in American English. For instance, dreamt (UK) versus dreamed (US), learnt (UK) versus learned (US), burnt (UK) versus burned (US), etc. In the case of dreamt, learnt, and burnt, they should all be pronounced with a t sound. In the case of dreamed, learned, and burned, they should all be pronounced with a d sound. I think in these cases here, the spelling is dictating which to use. If it ends in a t, it probably most likely should be pronounced with a t too. Hehehehe :)

Even still, sometimes in American English, there are two ways of saying a word. For instance, some people in America say they "texted" someone last night and some people say they "text" someone last night. With "texted", people say the word text (with a strong t at the end) followed by an "id" sound. Don't forget the "i". It is the same "i" as in the word "bit". Hehehe. With "text", it is just a final t sound.

Look at how "walked" is pronounced in American English too. Here it has a final t sound instead of a d sound like in "repaired". Plus us Americans swallow the l in "walked." Hehehehe :)

Just practice the verbs with roots ending in "t" or "d" -- these take the "-Id" sound. As far as distinguishing between when the suffix is pronounced "t" or "d," if you are pronouncing the final root consonant correctly, don't worry about the suffix sounding as "t" or "d" -- it will automatically come out just fine. (If the verb root ends in a vowel, then the suffix is pronounced as a "d," however, the English "d" sounds entirely different from the Spanish "d," so you might actually be better off using a "t," until you master the pronunciation of the English "d." I hope this helps. 

Thanks guys... both comments are helpfuly for me. Hope I can master it someday LOL