Complex is used to refer to the level of components in a system. If a problem is complex, it means that it has many components. Complexity does not evoke difficulty.
On the other hand, complicated refers to a high level of difficulty. If a problem is complicated, there might be or might not be many parts but it will certainly take a lot of hard work to solve.
"On to" implies that you are removing an object from one place to another, either physically, or conceptually. "Onto" refers to the positioning or placement of something, so that it physically touches the object. To help make this distinction, place a comma in the sentence.
I will go on, to the rock.
This tells the reader that the speaker plans to travel, until he arrives within close proximity of the rock. The speaker said nothing about making physical contact with the rock. Once the speaker arrives, they may declare,
I am going onto the rock.
At this time, the speaker indicates they are about to physically tough the rock, and stand upon it.
The "into" comment is also a matter of action versus position. Also, consider the prepositional context. This statement makes no sense, out of context:
I will go in, to the rock.
This means the person is going the the inside of something (cave, tunnel, etc.), to the location of the rock. More information about the location of the rock is needed.
Logically, one can not go "into" a rock, which implies the speaker is going to travel to the inside of the rock, for two reasons: first, there is no "inside" of a rock, as it is a solid; and second, the rock is kn fact solid. The concept of going into a rock is nonsense. Well, unless you're abusing hallucinogens.
However, liquids are different. You can logically state,
I am diving into the ocean.
I am walking into a cloud.
I am staring into the abyss.
In these cases, substituting "in to" just doesn't work, because these things are massive and are unable to be contained in a reasonably finite space. Therefore, saying you are "walking in, to the ocean" doesn't make much sense. If anything, you're far more likely to be "walking out, to the ocean." If you were already in the ocean, and the subject were walking toward you, from land, then this could well make sense, but is rarely the case.
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In modern English and in the HR sense, they are pretty much used interchangeably and most people assume they mean the same thing.
If you look at their etymology, though, "curriculum vitae" should technically describe what happened over the course of your life, i.e. who your parents were, when and where you were born, upbringing, education, etc., before going into your work experience, while "resume" should sum up your relevant experience and is arguably targeted to your audience, i.e. if it's for an IT job, you'd leave out that you worked as a waiter during your Uni years and you wouldn't think of mentioning your parents (unless maybe their names are Bill Gates or something).
So I would always call what I send for a job application a "resume", but recruiters in my space (IT) typically say "please attach your CV" and mean the same thing. I suppose which you use depends on how pedantic you want to be....