does suponer take subjunctive?
if so, does this mean " to guess something"?
It sounds like it would be similar to verbs like creer and pensar.
If you use creer to declare some fact or something very probable then it takes the indicative. If you use creer to express doubt (I think that it may, but I am not sure) then it is used with the subjunctive mood.
Same reasoning with pensar. If you are stating your opinion on a subject then it takes the indicative. If doubt is conotated then it is used with the subjunctive mood.
Same reasoning with suponer.
Supongo que tendré que hacerlo así. I'm declaring that I am going to do it that way.
Supongo que tengas razón. I suppose that you may be right.
Notice the distinction between obligation and suppostion
He is supposed to and I suppose (I imagine)
One of the most easiest ways of spotting if a verb is in a subjunctive form, is that you will almost always (if not always) you will find the nexus "que" after the verb. If you look at the examples above, all of them have "que" after the verb.
Supongo que sí... ¿por qué?
Does suponer take subjunctive?
Yes, Suponer takes the subjunctive
Wishes, desires, demands, expectations, suppositions (I suppose that...)
among many other characteristics require the use of the subjunctive
Please see Paralee's excellent lesson on the Subjunctive mood
Lesson 3 :1
... if so, does this mean " to guess something"?
Suponer means to suppose but it can also mean to imagine, too mean
For example: Suponer (tomar como hípotesis)
Supongamos que lo que dice es cierto = Let's suppose or assume what he said was true
Supongo que tienes rázon = I suppose that you're right
Suponer (atribuir + me, te,o le etc..)
Le suponia más edad = I imagined or thought he was older