ы даете совет другу. Используйте should или shouldn’t.

1)Your friend rides his bicycle at night without lights. You think this is dangerous. Advise him not to do it.   
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2)Your friend is going to visit Greece. Advise him to learn a few words of Greek before he goes

1)You shouldn't ride your bicycle at night without lights.2)You should lear a few Greek words before going to Greece.