The correct title should be: کی (چه موقع) غیر استفاده میشود؟ Yes, there is بی as well but it is the equivalent of the suffix "less" in English. There is no rule find out which negation to use for adjectives in Persian just like English! So the only way to learn them is to memorize them. EXAMPLES: For بی (the antonym of بی is با) 1- 1- با سواد : literate 1-2- بی سواد : illiterate 2-1- با ادب : polite 2-2- بی ادب : impolite For نا: 1-1- خوشایند : good 1-2- نا خوشایند : bad 2-1- خرسند : happy 2-2- نا خرسند : unhappy 3-1- گفته : said 3-2- نا گفته : unsaid For غیر: 1-1- قانونی: legal 1-2- غیر قانونی : illegal 2-1- طبیعی : natural 2-2- غیر طبیعی : artificial 3-1- واقعی : real 3-2- غیر واقعی : unreal 4-1 ممکن : possible 4-2- غیر ممکن : impossible (I think غیر can be used for the adjectives ending with an ی but I am not sure about this.) For نه: نخراشیده : nakharaashide : ugly ( this word is very informal)

actually you should know the root of the words (adjectives) you wanna use with that suffix, غیر is an Arabic suffix and you can not use it for Persian adjectives so غیر+ adjectives with Persian roots =non-existed and incorrect! غیر+adjectives with Arabic roots= correct (ex:refer to given answers) as our friends mentioned.

Examples for بی meaning "less" in English: 1-1- باک : fear 1-2- بی باک : fearless 2-1- سیم : wire 2-2- : بی سیم : wireless 3-1- دقت : care ( it actually means precision not care!) 3-2- بی دقت : careless

I think is hard to explain this question because it is a grammatical question and you would be confused,but don`t worry because if you read more Persian text,you would get it.you should learn it by reading and speaking,I mean learn it naturally.