How sure you are: 一定, 大概, 也许, 恐怕

规则 一定 = certainly, on evidence: 他学了十年，一定很好。
大概 = roughly, at a guess — often with a number: 大概三百页。
也许 = perhaps, either way is possible: 他也许知道。
恐怕 = I am afraid — ONLY for unwelcome news: 恐怕来不及了。 Never 恐怕你会成功。
不一定 = not necessarily, and it can close a sentence: 他来不来还不一定。