Por supuesto could be translated as "by assumption" or "by supposition", if that helps any. The Spanish "he" does not mean possession, but you'll hear "He aquí una carta" for "Here is a letter" that could be construed as "Tengo aquí una carta". I've never been 100% sure there is no connection.
spanishdict says that "suppose" is just one of several meanings of "suponer", and under the "to think" meaning, it has "suppose", "assume", "imagine".
Both "I have supposed it" and "I have assumed it" seem awkward. Using "so" instead of "it" seems more natural, and to me, "I have assumed so" seems more natural than "I have supposed so", although the present tense "I suppose so" and "I assume so" both seem equally natural.
The errors are only pointed out if people report the issues on the relevant link when they think DL has got them wrong. These discussion pages are just that, discussions among members. They are intended as a place where you can ask questions and discuss issues with fellow members, not to report problems to DL.
Um, I do both - I think that's what most people do. The issue is that, 9 months ago, the first poster pointed out the problem, and it wasn't fixed. And there are many posts like this... (interesting to note, btw, that Duolingo removed the timestamp from old posts). In fact, I stopped using Duolingo months ago, and only now am I getting notices that my suggestions were accepted. It's a nice program, and I'm glad it's free, but it's clearly a work in progress.
I first tried it months ago but got frustrated. After reading the Wikipedia article, I decided to try it again. This time, I have seen things actually get fixed, but, more importantly, I have learned quite a bit and, I hope, have helped some others. Now I sometimes try to blow away a heart with what I know is a correct idiomatic translation.
I restarted Duolingo recently after stopping in April, and I see they now have half-hearts with the opportunity to correct your mistakes. I really like that approach. I was tired of getting things "wrong" (according to Duolingo) and having to restart a lesson because I ran out of hearts.
Each language has only a handful of volunteers to moderate and change things. I am not sure if they follow the discussions at all--I think it depends on the individual. But considering the number of people using duolingo, I can only imagine how many reports each volunteer has to wade through and possibly research before changing anything. Personally I am impressed that we have such a wonderful tool for free. Yes, it has a few drawbacks, but it is always being improved, and people are giving their time for our benefit. These comment areas are a wonderful place, as well. So even when the changes take a while, we continue learning. I am glad you are continuing with your studies.
"I have assumed that for some time" (present perfect) is somewhat different from saying "I did assume/ I assumed that at one time, but no longer do (simple past).
That is the difference between simple past and past perfect. http://www.englishpage.com/verbpage/pastperfect.html had studied http://www.englishpage.com/verbpage/simplepast.html studied
Also, see: http://www.englishpage.com/verbpage/presentperfect.html
That's the answer I gave and it was marked wrong despite the fact that this was one of the hints. What's frustrating is that the answer given is not one of the hints. Duolingo is definitely a work in progress, just like my Spanish learning. The problem is I feel like the blind is leading the blind here. I have no qualms about admitting that my Spanish is limited, but when a program like Duolingo tells me that one of the "right" answers is wrong, and gives me a totally different answer, one not given in the hints, I start having doubts that Duolingo.can teach me how to speak Spanish.
Here are some authors that used it along with the Spanish translation (sometimes you have to copy and paste these links to get them to work): http://www.linguee.es/ingles-espanol/traduccion/i+have+meant.html
There's a reason. Supuesto means false in the sense of "Tu supuesto amigo!" = "Your so-called friend!" That definition doesn't make any sense here any more than if you used "I have budget it" for "Lo he presupuesto". Part of the trick is knowing which of the drop-downs apply. Sometimes, none do.
If it just said "false" it would be ok as a hint that doesn't happen to fit here, but it's wrong everywhere to have "(I) have false (it/him)" as though "false" is a past participle. I think it should be reported as "the dictionary hints on hover are wrong or missing".
If the computer can't match it to the list, Duo doesn't mark it right. That doesn't mean it can never be translated like that. I can't think of where I'd translate this particular example using "guess", but "supongo" can certainly be "I guess". In the US, we often use "I guess so" in place of "I suppose/assume/presume so", especially to express more uncertainty.