Confused in choosing でした or です in のは～だ grammar

Question

Which one is the correct one between the following 2 sentences? Are my English translations correct?

A: 昨日食べたのは肉まんです。

A': It is nikuman (steamed meat bun) that I ate yesterday.

B: 昨日食べたのは肉まんでした。

B': It was nikuman (steamed meat bun) that I ate yesterday.

Note: I am not a native speaker in both English and Japanese.

Answer 1

The key here is the word order. The neutral order is “昨日肉まんを食べました”. By saying it in the order “昨日食べたのは肉まん[です/でした]”, you are already conveying a contrastive nuance. That is to say, you ate 肉まん and not something else.

です/でした tends to have implications on this contrastive nuance:

昨日食べたのは肉まんです。ラーメンではなくて。
It was nikuman that I ate yesterday. Not ramen.

昨日食べたのは肉まんでした。ラーメンではなくて。
It turns out it was nikuman that I ate yesterday. Not ramen.

Using でした can imply that you are overturning a previous belief or statement. You previously thought/said it was ramen, but it turned out to be nikuman.

So for example, these are the natural choices:

• 先に帰ってきたのはお父さんです。
  It was my father who came home first.
• お母さんが先に帰ってくるはずだったのに、先に帰ってきたのはお父さんでした。
  My mother was supposed to come home first, but it turned out to be my father who came home first.

Answer 2

As Toshihiko wrote, both A and B are correct sentences in Japanese. However, they are not always interchangeable.

For example, suppose that you ate nikuman yesterday, but today your coworker asked you:

昨日カレーまんを食べていましたよね。どこで買ったんですか? You were eating karēman (curry-flavored pork bun) yesterday. Where did you buy it?

You can say (A) いいえ、昨日食べたのは肉まんです in response, but it is strange to say (B) いいえ、昨日食べたのは肉まんでした. It is hard for me to pinpoint why (B) sounds strange in this case, but I feel that (B) emphasizes the pastness of the event for no reason even though you are making a correction now.

Answer 3

Both A and B in Japanese are correct, the same fact as A' in English. B' is incorrect English. Do you want to know about English?