Usage of نى: With Participles
نى» قوشۇمچىسىنىڭ ئىشلىتىش ئۇسۇلى: سۈپەتداش بىلەن»
In the majority of cases, nouns modified by perfect/imperfect/continuous participles (e.g., "the book that has been read", "the bus that is going to arrive", or "the plane taking off") will require the accusative نى when used as a direct object, with the suffix placed on the noun modified. This should not be surprising, since such participle modifiers tend to make the object very specific, and thus definite.
However, participles, like adjectives in general, can also be vague in nature and can thus modify nouns in an indefinite manner (thus requiring that the نى NOT be added). Compare the following two (fairy-tale themed) sentences:
In the first sentence, the horses that the boy looks after are not definite - thus leading the reader to assume that we are probably dealing with a context or a world where there are many flying horses and (likely) many boys charged with the task of looking after them. The second sentence uses the accusative نى and makes the horse(s) definite, suggesting that there is probably only one such horse in existence, that it is special and unique, and the boy is the one tasked with looking after it.