I try to speak clearly. I would say "I would do it". If I used the contraction "I'd do it" I would sound both "d's". even though it would sound very similar to "I do it". For me, there would be a little difference in speaking. The shop "close down" is baby-talk. You should hear that "closed" is past-tense. It might be a very slight difference, but the speaker would be attempting to sound both 'd's'. "I can't take it"....there would be a short break between the first and second 't'.

Time After Time, There are three factors that determine the pronunciation of an English word. -The pronunciation of the individual letters. -The liasons of the letters with other letters within a word, or with other words in a sentence. -The placement of the word in the sentence, that is, whether it is stressed or unstressed. I DO....... I'd DO it......The tongue is held on the roof of the mouth just a fraction of a second longer. close DOWN.... clozdown closed DOWN...clozədown The "z" sound is made and the breath released with the "d". Normal speech I can DO it......... [I kən do it] positive.....The a is not pronounced and "do" is stressed. I CAN do it......... [I kææn do it] extra positive....The a is held longer and "do" is unstressed. I CAN'T DO it. ........[I kæn(t)do it] negative........The n is held and the breath is released with the "t". Both "can't" and "do" are stressed. I CAN'T do it........[I kæn(t)do it] extra negative ......Only "can't" is stressed.

As far as actually SPEAKING English is concerned, one should let to words flow, not try to pronounce every sound. English isn't like that (Spanish is!). So as far as speaking fluent English is concerned, there is no detectable difference in the sounds.

Yes, the pronunciation is different. You'd hold the 'd' slightly for "I'd do". No decent speaker of English would confuse "close down" and "closed down". The context of the whole sentence tells us what to hear. You'll only have a problem with can/can't if you pronounce the vowel sound the same. You'll hear that British speakers (at least) use completely different sounds depending on whether it's a negative or not. No confusion there. Even for the speakers who do pronounce the "a" in can/can't the same, you'll still hear a slight holding of the 't' in "can't talk".

Ye, there is a difference.. it's not that obvious, but it does exist. Try to write them here: http://translate.google.com/ and listen to the difference yourself :)