詢問有關英文sentence問題

The first sentence is correct.

The second sentence is also correct, as simple past tense could be used to tell some action was true for a duration in the past; such as:

Mary removed the blockages from the tunnel for an hour yesterday.

It could also be rewritten as:

(1) Mary removed the blockages from the tunnel yesterday. (a completed event in the past)

(2) Mary removed the blockages from the tunnel at 8:00 a.m. yesterday.
(she started removing at 8:00 a.m. until completion)

(3) Mary was removing the blockages from the tunnel at xxxx yesterday. (where xxxx could be any time from 8:01 to 8:59 a.m.)

Both sentences are correct.
1) present continuous tense - is / am/ are + verb + ing plus now
2) simple past tense - whatever happened in the past like yesterday, last week, last month, last year etc.

ok無問題 1 瑪麗移除阻塞從隧道現在
2 瑪麗從 8:00 上午至 9:00 上午昨天的隧道中刪除淤塞 一個有時間,一個冇

Mary is removing blockages from the tunnel now. 對！

Mary removed the blockages from the tunnel from 8:00am to 9:00am yesterday. 錯！

應是：

Mary removed the blockages from the tunnel at 8:30am yesterday.

or

Mary was removing the blockages from the tunnel from 8:00am to 9:00am yesterday.

你有無睇過：

https://hk.knowledge.yahoo.com/question/question?qid=7014022400049