Study this example :
Lisa : Why don’t you buy some nice clothes?
Lisa said to Mary, “Why don’t you buy some nice clothes?”
Lisa suggested that Mary buy some nice clothes.
In this example, buy is the subjunctive. The subjunctive is always the same as the base form (I buy, he buy, she buy, etc.):
I / he / she / it | do / buy / be, etc.
we / you / they | do / buy / be, etc.
We use the subjunctive after these verbs:
I insisted he have dinner with us.
The doctor recommended that I rest for a few days.
John demanded that Lisa apologize to him.
What do you suggest I do?
We also say It’s essential / imperative / important / necessary / vital (that) something happen:
It’s imperative that the government do something about health care.
You can also say:
It’s essential for everyone to be at work by 9:00 tomorrow morning.
It’s imperative for the government to do something about health care.
The negative is not + base form (I not be, you not leave, she not go, etc.):
The doctor strongly recommended that I not go to work for two days.
It’s very important that you not miss this appointment with your eye doctor.
You can use the subjunctive for the present, past, or future.
I insist you come with us.
They insisted I go with them.
Note the subjunctive be (often passive):
I insisted that something be done about the problem.
It’s essential that this medicine not be taken on an empty stomach.
The airline recommended we be at the airport two hours before our flight.
Other structures are possible after insist and suggest:
They insisted on paying for dinner.
It is a beautiful evening, so I suggest going for a walk.
You cannot use the infinitive (to …) after suggest or insist:
She suggested that he buy some new clothes. (not suggested him to buy)
He insists on going with us. (not he insists to go)