一定的英文：
surely
certainly
necessarily
fixed
a certain (extent, etc.)
given
particular

参考例句：

The new show is certainly pulling in the crowds.
新的表演一定很吸引人。

Make sure the top is screwed back tightly onto the jar.
一定要把罐子盖拧紧。

I'll help you insofar as I can.
我一定尽力帮助你。

The party on Saturday night is going to be awesome. Booyah!
周六晚上的聚会一定很棒，哦耶！

You must copy it fair.
你一定要把它抄写清楚。

To wander from a set course or point of attention; stray.
离开一定的路线或注意点;游离

Take his vital signs at regular intervals.
每隔一定时间检查他的生命体征。

A lady who is house-proud must also like to keep open house.
一个讲究家里摆设的女人也一定很好客。

The effective permeability is stated as some numerical value at some given saturate conditions.
有效渗透率用在一定饱和条件下的数值表示。

He must have hit the jackpot with the sales of his last gramophone record.
他卖掉最后的一张留声机唱片，一定赚了大钱。

surely是什么意思：
adv. 当然，无疑

brassard: "Sure, sure."
布拉萨尔:“当然，当然。”

Make sure you get there in time.
一定要准时到达那里。

Be sure not to let the truth slip.
千万别把真相泄漏出去。

certainly是什么意思：
adv. 无疑的，肯定的；当然，行

You are certainly a profiteer.
你真是个奸商。

That ring is certainly the cheese.
那只戒指无疑是精品。

You are certainly a talented artist.
你当然是位有天赋的艺术家。