Rule 1

You may end a sentence with a preposition. Just do not use extra prepositions when the meaning is clear without them.

tick1 That is something I cannot agree with.
tick1 How many of you can I count on?
tick1 Where did he go (to)?
tick1 Where did you get this (at)?
tick1 I will go later (on).
tick1 Take your shoes off (of) the bed.
tick1 You may not look out (of) the window.
tick1 Cut it (up) into small pieces.

Rule 2

Use on with expressions that indicate the time of an occurrence.

He was born on 23 December.
We will arrive on the fourth.

Rule 3

should never be used in place of have.

tick1 I should have done it.
incorrect I should of done it.

Rule 4

refers to two. Among is used for three or more.

Divide the money between the two of you.
Divide the money among the three of you.

Rule 5

implies entrance, in does not.

Sally walked into the office.
Sally was waiting in the office.
Cut the cake into six slices. (The knife enters the cake.)