In german how do you decide between accusative and you

The nominative case is used for the subject of the sentence. The accusative case is used for the direct object in a sentence. For example,

"Der Schriftsteller schreibt einen neuen Roman."

The subject of the sentence is Schriftsteller; this noun is used, therefore, in the nominative case. "What is the author writing? - A new novel." The noun Roman is the direct object; that's why it is used in the accusative case.  

Another example:

"Der Mann liest den Brief."

Der Mann is the subject (nominative); den Brief is the direct object (accusative).    

hello i am m

Hi There!

Question 1: Ich habe keinen bleistift. (I do not have a pencil).

Question 2: Wir haben keine katzen. (We do not have any cats).

Question 3: I not to sure because they all look good. (You do not like swimming).

Question 4: Peter hat nicht haustiere. (Peter does not have pets).

Hope This Helps :)