Use the infinitive, as in English.

Click here for Infinitive Paradigm.

parainw  umin  apelqein.

"I advise you to go away."
 

ekeleusamen  autouV  mh apelqein.

"We ordered them not to go away."

In English we use a complicated expression, "I prevent you from doing something."

But in Greek, it's quite simple.

Verbs of Prevention work like "forbid" and simply use the infinitive: "I forbid you to do this."

apagoreuw  umaV  apelqein.

"I forbid you to go away."
 

ekwlusa  umaV  apelqein.

"I prevented you from going away."
 

Sometimes the Greeks add the word mh  before the infinitive, but this does not alter the meaning:

apagoreuw  umaV  mh  apelqein.

"I forbid you to go away."

If you are not preventing someone from doing something, then you put mh ou before the infinitive.

ouk  apagoreuw  umaV  mh ouk  apelqein.

"I do not forbid you to go away."
 

ouk  ekwlusa  umaV  mh ouk  apelqein.

"I did not prevent you from going away."