Verbs of perception and some other verbs (hören, sehen, spüren, lassen, others I can't think of off the top of my head) can take a dependent infinitive, sort of like modal verbs.
As in English, the subject of the infinitive (the person or thing performing the action)
is usually in the accusative case.
There are some finer points (a lot, actually), and I don't know if Duolingo teaches them: z.B., Ich lasse mir die Haare schneiden when someone has something done to/for someone, including himself, or Wir helfen dem Kind lesen where the principal verb takes an object in the dative case.
I skipped these lessons, so I don't know for sure, but doesn't Duolingo give you a page of grammar before you get to the exercises? At any rate, you should get a German grammar book for reference while you're learning. Duolingo is a good beginning, but really just that--a beginning!
While there are definitely nuances between the two, neither past progressive nor simple past imply you saw (or didn't see) the whole thing.
"I saw him run in the marathon." = "I saw him running in the marathon."
Thank you for your answer. I was refering to cases where one is the whole thing and the other is a glimpse. Here are some references: http://www.pearsonlongman.com/ae/azar/grammar_ex/message_board/archive/articles/00066.htm http://dictionary.cambridge.org/grammar/british-grammar/verb-patterns/hear-see-etc-object-infinitive-or-ing
I understand now that in some cases, both forms mean "seeing part of the progress". That leaves me a bit confused, though keywords may help I guess (i.e.: marathon, hard to observe the whole thing)
My question was how to express the nuance (when there is) in German.