Could you also use the past-participle of sehen, or does that change the meaning?  Instead of "ich habe ihn dem Pferd einen Apfel geben sehen", could you say "ich habe ihn dem Pferd einen Apfel geben gesehen"?

actually, I guess "ich sah ihn dem Pferd einen Apfel geben" probably comprises Haupt- und Nebensatz.

In English, you'd say "I saw him give the horse an apple" or "I saw him giving the horse an apple".  I'm not it's possible to make that any easier.

Although it's possible to translate this relatively compact sentence into German using solely a main clause (Hauptsatz), e.g. "ich sah ihn dem Pferd einen Apfel geben", I suspect it becomes increasingly challenging to use only a main clause when the sentence becomes richer, for example "I saw him trying quickly to remove all the traces of a messy fried breakfast from his clothes".  The English structure stays roughly the same (excepting the adverbs, adjectives and indirect objects) but the German one has to split into a Hauptsatz / Nebensatz combination.

If you try to use higher grammar you say:

Ich sah ihn dem Pferd einen Apfel geben (so more or less what scott-k wrote, but as in english in the past tense "saw"). Alles andere sind Vereinfachungen (people don't use the complicated grammar when they speak, as in every language). Im curious how exactly you would easy it up in english: I saw him when he gave... or how he gave...?????

ah, it makes sense now...  Thank you