When different subjects perform the similar actions, we use the words so and neither for positive and negative respectively. The benefit is that it saves our words without changing the meaning. Let’s understand this with the help of an example.

Ram works very hard.
Shyam also works very hard.

Now when we have to talk about both Ram and Shyam, we can say

Ram works very hard, so does Shyam.

In the second part of the sentence, we have not used the verb ‘work’ because we have used it in the first part and same is conveyed with the help of ‘so’. Plus we have used ‘does’ because with a single name, we use ‘does’. Take some more examples.

I live in this colony.
My friends also live in this colony.

We can combine the above sentences by saying.
I live in this colony, so do my friends.
Here we have used ‘do’ instead of ‘does’ because the subject (my friends) is plural and with plural, we always use ‘do’

Now lets see how to use this in the negative sentences.
I don’t travel by train,
My sister doesn’t travel by train.
The above two sentences can be combined by saying
I don’t travel by train, neither does my sister.

We don’t like to fight with others.
Our neighbours don’t like to fight with others.
We don’t like to fight with others, nor do our neighbours.

From the above examples, it is evident that in the negative sentences, we use ‘neither’ or ‘nor’ instead of ‘so’.