In English, we have words like “from”, “to”, “with”, etc. These are called prepositions. They exist in Arabic, too–“huruful jaar” (حُرُوقُ الجَرّ).
Huruwful jarr modify the words immediately after them, which are called “majruwr” (مَجرُور).
In Arabic Grammar, majurwr words always take kasra (either single or double).
(The one exception is feminine names, which take fatha.)
So for example, if we have a book (kitaabun), then we add the preposition with (bi), it becomes “bi kitaabin” (بِ كِتَابٍ). Or if we have the car (as-sayaaratu), and we add the preposition on (3ala), it becomes “3ala as-sayaarati” (عَلَى السَيَارَةِ).
Wallahu ‘alim. There are only about 17 huruwful jarr, so memorize them inshaAllah. May Allah give us a persistant understanding of this simple but crucial element of Arabic Grammar, ameen!
fiy (فِي): in
ila (إلَ): to (eg. travel to)
3ala (عَلَى): on (top of)
min (مِن): from
li (لِ): for (eg. for you)
ma3a (مَعَ): with
bi (بِ): at/in/with
7atta (حَتَّ): until
munthu (مُنذُ): since
3an (عَن): about
ka (كَ): like/as
 Faynan, Rafi’ El-Imad. The Essential Arabic: a Learner’s Practical Guide. New Delhi: Goodword Books, 2004. 23-24.