Thank you, Eric :) Your question made me curious and I looked for the answer for both of us; I honestly didn't know any rule except that it's easier for me to use (ب/ في) since Arabic is my mother tongue. When to use (bi/ بـِ) : 1. When you hold something for real or attach it with the object in the word after it: أمسك بيدي: Hold my hand إصطدم بالحائط: He bumped into the wall 2. When using something: رسمتُ بالفرشاة: I painted with a brush. 3. When explaining why: بجريمتك دخلت السجن: You went to prison because of your crime. 4. For recoupment: اشتري الكتاب بدولار واحد: Buy the book for one dollar. 5. For companionship: "I know that companionship is not the right word here, yet I think you'll understand what I mean when you read the example". إذهب بأمان الله: Go and Allah is with you/ you go with Allah's safety. 6. To swear by: بالله عليك أن تجلس: For God's sake, will you please sit down? When to use (fi/ في) : 1. In sentences with real/ metaphor situational or real place: أقمتُ في الفندق: I stayed in a hotel. في الحياة الكثير من المحن: In life there are many ordeals. 2. It's also used for explanation: دخلت النارَ إمرأةٌ في هِرَّةٍ حبستها: A woman went to hell because of a cat that she tortured.

(في) is used to indicate the existence of an entity within a defined container. The container can be spatial (box, house, neighborhood, country, solar system or even universe) temporal (second, hour, day, week or even a century) or imaginary (dream, idea, mind, science, category or even speech). (ب) is used to indicate the cause of an effect (in the meaning of "by the use of", "by the effect of" or "by the assistance of"). EX: في يدي فرشاة In my hand a brush بيدي اكتب With my hand I write

simply u can use fi as "in" in English and u can use bi as "with" in English :) Exceptions are mentioned in the comment above :)

Both somehow have the meaning of "at, in ". But there are some situations when we can't apply this on them. like verb + adverb. for example" I used it wisely" استخدمته بحكمة I ate quickly أكلت بسرعة You see the deffirences in meaning no ?.