可以不加，也可以加in，例如：
1、Why should I believe you? Everything you ever told me was a lie.我爲何要置信你你所說的一切都是謊言。
2、I believe competent authorities will handle the case involving the21 Indians youmentioned according to law.你提到的21名印度人在深圳的問題，相信有關部門會依法處理。
3、I believe we will create the splendor for Xineng day by day.我相信，以後的每一天都能創造錫能的輝煌！
