(2016-04-16, 9:57 pm)hazwanium Wrote: why is that in suru verb, the particles wo is sometimes omitted and sometimes it's not
what is the difference between benkyou suru and Kyanpu O Suru. Can i just write kyanpu suru and omit the particle Wo, does it means the same thing
It means the same thing, but you can only omit the wo some of the time.
Sometimes a given 'nounwo suru' term also will have a dictionary entry for the noun that indicates it's a 'suru verb' as well. In such cases, you can simply omit the 'wo' whenever you like. (There are also "nouns" that exist only to serve as "suru verbs" ... they'd logically be nouns if they existed independently but they never do. In such cases, you never use 'wo' with that term.)
So, nouns that do -not- appear in dictionaries as also being suru-verbs, you are supposed to always put 'wo' with them, but if you drop the 'wo' the meaning will not change. However, for such terms you shouldn't drop the 'wo' unless you're speaking extremely casually (using plain form and dropping lots of other particles as well).
As it happens, benkyou and kyanpu both exist as suru-verbs so you don't need to use the 'wo'.
Most 'noun+wo+suru' phrases that are very common are also suru-verbs (it tends to happen over time that it becomes more acceptable to drop the 'wo' the more established a given suru term is). Words that are most often used as nouns but can infrequently be made in to verbs with 'wo suru' are the ones that will 'require' (for 'proper' speech and writing) the 'wo' ... in such cases, the use of the particle helps grammatically clarify the unusual use of the word. Which, of course, doesn't stop it from being dropped in extremely casual speech.