Difference between трудно и сложно.
As the title says, and if there are different situations in which I would or should use one over the other.
While we're at it, how would I ask what the difference is between something, in Russian?
В чем разница между .....? В чем состоит/заключается разница между..... Есть ли разница между.... ? И где же разница между....?
I think "трудно" is more like "hard" and "сложно" is similar to "complicated".
They are fairly close but their connotation occasionally make one of them more suitable in a certain situation.
The adjective трудный, based on труд ("work, effort"), implies the thing is "hard" in that it takes effort and commitment or is physically/emotionally hard.
Сложный is more like complicated in the sense it is not simple and there are many variables to consider. It requires skill and knowledge; maybe you do not even know where to start. Трудный also works, adding a connotation of such a task being demanding from the emotional standpoint.
So they are indeed close. However, if it is hard for you to breathe, трудно or тяжело (lit. "heavy") describe the situation much better than сложно. In most situations it is up to you to decide which shade of "hardness" is appropriate.
Of these particular words, сложно/трудно (adverbs or predicate adjectives), "трудно" is several times more popular, which is not much but guarantees that "трудно сказать" is more common than "сложно сказать", even though both are possible.
In general, трудный and сложный are both very common.
Possibly the best explanations I could have asked for. Thank you all.
In my opinion, the adverbs сложно and трудно are very similar, so forget about the difference. The adjuctives трудный and сложный are not so common, because there are stable collocations with defined words, like сложный процент (financial: compound interest); сложная фунция (mathematics: complicated function). Nobody says трудный процент, it is a mistake.