I seriously must have missed something along the way since the 'clitics' section... as I instinctively put "I suppose it is" - but feel I'm out of my depth a little... I think we are expected to know more than we have been taught, at this point, and for the previous few sections. But maybe it's just me?! Sorry for the clutter, but wanted to say this.
It belongs to a group of irregular verbs that are modeled after "porre" (to put, place, lay). http://www.wordreference.com/conj/ITVerbs.aspx?v=porre
If you want to say "I suppose that it is so" this complicates things a little and this type of phraseology is dealt with in Duolingo under the sections covering subjunctive verb use. The way to say "I suppose that it is so" would be "Suppongo che sia così" (Literally "I suppose that it be like this")
So the only way to translate "Suppongo di sì" is by "I suppose so"
In a little more depth: The subjunctive is used to convey thoughts, hopes, suppositions (as in the above.) It relates to an older style of English often seen in Shakespearean literature. The key to it is that it reflects the word "be" as in "what be the problem?" rather than as we would now say "what is the problem?" You can often see it coming by the use of the word "che" to mean "that" - then you know to go into the subjunctive of the verb.
Porre (like supporre, proporre, esporre...) is a contraction of Latin "ponere", as you can sere eg looking at the imperfect tense, where it behaves like a regular second conjugation verb. In most other tenses it's irregular and (as a rule of thumb) you have to put an 'o' where a second conjugation verb has an 'e'. It won't work everywhere but it may help..
I think so, I suppose so, I believe so, I guess so.........All these phrases have subtly different meanings when used in different contexts. The problems is they are used interchangeably in everyday speech. In the US "I guess so" is dominant and in the UK "I think so" is mostly used.However in the UK "I suppose so / I believe so" has also been captured by educated speakers to denote status (watch BBC interviews who also use "extraordinary" incessantly). Learning Romance languages helps to re-introduce subtlety and precision in grammar and discourse.