simple past or past perfect?

<<He fell asleep when I had given my speech.>> shows that the man fell asleep as soon as the person had given the speech. <<He fell asleep when I gave my speech.>> doesn't make it clear (to me) if the man fell asleep while the speech was being given or after the speech was given.
I'd rather use 'after' in place of 'when' - especially if he fell asleep after the speech was given.

<<He worked until it had rained.>> seems grammatically incorrect and/or unclear. To say, "He worked until it rained" makes it clear what the person was doing up to the time it began to rain. The word 'until' is both a conjunction and a preposition.

http://esl.about.com/library/grammar/blgr_adverbclauses_time.htm

Until, till We waited until he finished his homework.
I'll wait till you finish.
'Until' and 'till' express 'up to that time'. We use either the simple present or simple past with 'until' and 'till'. 'Till' is usually only used in spoken English.

http://www.englishpage.com/verbpage/pastperfect.html
FORM Past Perfect

[HAD] + [PAST PARTICIPLE]
Examples:
I had studied a little English when I came to the U.S.

They had never met an American until they met John.

NOTE: When you are using a verb tense with more than one part such as Past Perfect (had met), adverbs often come between the first part and the second part (had never met).

What is the difference:

I had studied a little English when I came to the U.S.
I studied a little English when I had come to the U.S.

They seem to say the same thing, the first one just sounds like better English to me (But remember this is coming from someone who isn't a native English speaker).

I agree because I am not a native English speaker too.

I agree, and I am a native speaker.

Anyway, for the main question, my two cents:

>>Ex1: He worked in the field until it rained.
>>Ex2: He worked in the field until it had rained.

I agree completely with the person who said the first one means he stopped working before it rained, and the second one after. Since it doesn't make much sense, though, a native speaker probably wouldn't use Ex2 without more there as context, e.g.: "He worked in the field until it had finished raining." or "He worked in the field until after it had rained."

In any case, the "had" in Ex2 indicates completion. It's more clear when constructed: "He didn't stop working in the field until it rained" vs. "He didn't stop working in the field until it had rained [an inch]."

I agree to "They SEEM TO say the same thing...", but actually they don't, of course. How could the two sentences are of the same meanings?
Ex1: I had studied a little English when I came to the U.S.
Ex2: I studied a little English when I had come to the U.S.

Ex1 means I studied a little English and then went to the U.S., while Ex2 means I went there and then studied a little English. They are totally different.

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As for the Until structure, I just wanted to point out the stoppage that most grammars have not mentioned, as in the above explanation from esl.about.com:
>> 'Until' and 'till' express 'up to that time'. We use either the simple present or simple past with 'until' and 'till'. 'Till' is usually only used in spoken English. <<

If we understand the stoppage, its example is incorrect:
Ex: We waited until he finished his homework.
== It should have been "had finished". The verb FINISH doesn't exempt itself from obeying the grammar.