要翻译不定式作表语，首先要理解不定式作表语的用法。

不定式作表语的用法主要可分两种情况：

用法一：不定式用作表语不具情态意义，此时的不定式只是表示主语的内容（主语与表语可以划等号）：

My hobby is to collect old coins. 我的嗜好是搜集古币。

His aim is to do two years’ work in one. 他的目标是一年干两年的工作。

Her job is to book bands for the festival. 她的任务是为节庆活动预约乐队演奏。

My one idea was to get out of that place. 我唯一的想法是离开那个地方。

Our aim is to provide greater choice for consumers. 我们的目标是为消费者提供更多的选择。

His principal interest in life was to be the richest man in Britain. 他生活中的主要兴趣是成为英国的首富

My first reaction was to hit him, but he was old. My second was to resign. 我的第一反应是揍他，但他年纪已大。后来想到的是辞职。

用法二：不定式用作表语具有情态意义，此时的不定式可以表示“可以”“可能”“应该”“计划”“安排”“想要”“将要”“注定”“必须”等语气（有人将这样用的 be to视为半情态动词）。如：

He is to be shot at dawn. 定于黎明将他枪决。

She is to star in a new film. 她将主演一部新影片。

I cannot accept that he is to blame. 我认为不能责怪他。

You are to blame rather than he. 受责备的不应该是他，而是你。

She is to be buried in the family vault. 她将安葬在家族墓穴里。

He must study if he is to pass his exams. 他要想通过考试就必须学习。

We are to discuss the report next Wednesday. 我们将在下星期三讨论这个报告。

He needs to show a change in attitude if he is to succeed. 他要成功就必须表现出态度上的转变。

If we are to succeed in this enterprise, we shall need to plan everything very carefully. 我们要是想把这个企业办好，每一件事情都要认认真真地去计划。

回到网友的句子，其中的不定式具有情态意义，可理解为“可以”：

The arugula (芝麻菜) was to make a nice green salad, rounding out a roast chicken dinner.

翻译：芝麻菜可以用来做出美味的绿色沙拉，使一顿烤鸡晚餐变得圆满（给一顿烤鸡晚餐画上圆满句号）。