Question

When do we use V1/V1+s/es ??

Answer 1

We use v1/v1+s/es if we are talking about present tense 
v1 =for subject i,you ,they ,we
v1+s/es=for subject he,she ,it.

Answer 2

When you use simple present tense.

Pattern of simple present tense :

Subject + auxilary + V1

Example for V1 :

I go to school by bike --> we use 'go' because the auxilary of I is 'do'

I, you, they, and we use 'do' for auxilary

Example for V1es :

She goes to school by bike--> we use 'goes' because the auxilary of She is 'does'

He, she, it, use does for auxilary.