1.  A preposition always comes before a noun and it does not come before a verb.

  2. The noun following the preposition is changed from nominative  case to genitive case. This means that the last letter of the word will be changed from a damma to a kasrah.
 
 3. We know that most nouns in Arabic can have any of the three vowel marks. e.g “ the class”can be : اَلْفصْلُ  (al faslu), اَلْفصْلَ (al fasla), اَلْفَصْلِ (al fasli) with no change in meaning. But after a preposition the noun carries a kasrah in the end.

4.  The above sentence “ The teacher is in the class “ is to be translated as : اَلمُدَرِّسُ فِي الفَصْلِ    ( al mudarrasu fi al fasli). Therefore the noun coming after the preposition gets kasrah as ending.

e.g.         الْبَيْتُ    al-baytu         The House
             
            في الْبَيْتٍ  fi-lbaytin     In the house 
 
The Last letter of the noun that comes after the preposition is in kasrah. 
 
 some common preposition  in the Quran.

1. ' عَلَى   alaa  - means “ ON “

  ِالكِتابُ َعاى المكتب     Al-kitaabu ala-l maktabi      The book is in on the desk
 

2. مـِنْ  min   - means “ FROM “ 

مِنْ بَىْتٍ.  min baitun - from a house

3. 'اءِلَى - ilaa - means “ TO “

 إِِلَى 'الْبِىْتِ.  - ilal bayti - To the house

4.  فيِ - fi- means “ IN”

الماءُفي الكاَس - al-maufi l-kas - The water is in the glass.