▲推荐答案

1. They arrived earlier than I expected

2. They arrived earlier than I had expected

以上两个句子都是正确的。但是第一句不如第二句正式。

▲原因解析

1. 标准的句子应该是第二句，但是老外们有时不够严谨，所以造成过去时代替完成时情况的屡见不鲜。正如一个美国佬所言： Using “...than I had expected” is certainly correct. Many native speakers aren't always that careful with their tenses, so you can often hear them using versions like the first version.

2. 在口语交流中，had的发音有时几乎听不见，导致它与过去时几乎混同，所以老外们有时索性就用过去时来代替完成时了。 请看这个英国佬的观点：When it is contracted in speech it is almost inaudible if the word after 'd starts with a consonant, like“.... than I'd（这里的d几乎听不见了） thought ”, so if it isn't necessary, why bother at all

▲善意提醒

在比较正式的文体中，或者在考试中，我们提倡严格遵照规范。

▲祝您进步

Hope it's clear enough. Good luck.