According to the grammar explanation in a textbook I use, when you place "khi nào" at the end of the sentence it implies past tense and when you put it at the beginning it implies present/future tense, so I thought that "Anh ấy ăn bánh mì khi nào?" should be translated as "When did he eat bread?". However, that translation was marked incorrect so now I am confused. Can you use khi nào at the end of the sentence and still retain the present meaning after all?
You have a good explanation in the "Tips and notes" section" of this module:
"Using when - khi nào"
"Khi nào can be placed at the beginning or at the end of the question without changing any meaning. Preferably, our answer database has more questions containing khi nào at the beginning so you are recommended to follow."
"E.g: Khi nào bạn ăn bữa sáng? (When do you eat the breakfast?)"
You also will see some complements, descriptive markers and tense markers for verbs in the Past skill, and others like Common Phrases 2 and Common Phrases 3.
"vừa mới: this phrase is used in this formula: subject + vừa mới + verb, demonstrating that someone has just done something. This phrase indicates action occurred the past but only a moment ago. Note: Vietnamese people love to talk about what they just did. You are recommended to add this phrase to your vocabulary list." (Tips and notes, Common Phrases 2)
If you prefer to see a more voluminous description of the Vietnamese grammar, here is a text about it (especially chapters 8 to 10):
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