Question about ない form & Dict form

[gryffindor]

Hi guys,

Is the ない form the negative form of the dictionary form?

For example;

いく　(Dictionary form-- Positive) いかない (Dictionary form -- Negative)

If they are similar then why is the ない form always listed as such instead of the negative form of the dictionary form?

Am I right to say that both forms above are impolite and used when conversing with friends and colleagues?

My sense told me that we can add んです to make the two forms polite.

For example,
いくんです
いくないです

If that's the case, why don't we just simply use the ます form, which is polite?
いきます
いきません

What is the main difference between adding んです and ます?

[Joey]

ます form is the same as the dictionary plain form
いく　＝　いきます
adding んです is something differenty. the ん is short for の as is used at the end of sentences to explain something (sorry can't be more specific.) You should be able to read more about the explanatory の here:
http://www.guidetojapanese.org/particles3.html#part5