"My friend and I ate lamb for dinner yesterday."
I purposedly left le out in this sentence because zuotian already implicates past tense. Does anyone know why is it still incorrect?
Given that no other context was given, Duo should have accepted your answer. Merely the eating action happened in the past does not necessarily impose use of 了. There is nothing wrong if we say 昨天晚饭我吃魚，我的朋友吃羊肉, simply just to talk about the choice of food.
We use 了 when we want to indicate a change of state, which is "lamb eaten" in this sentence. In the following scenarios we would want to say it:
- Lamb being a specialty of the season or a dish that is not usually consumed. We want to highlight that "lamb eaten" appeared to you, as it is not normally expected;
- Your friend got poisoned and in search of the possible cause you explain to the doctor what you ate, so highlighting that "lamb eaten" appeared to you.
This sentence has no problem grammatically. It is somehow less common to say 羊肉晚饭 which is equivalent to "lamb dinner". This make it a little not very natural. Some restaurants may offer set dinners and call them with their principle ingredient. In those cases the word 晚餐 is usually used (i.e. 羊肉晚餐).
is there any differences between “...吃了羊肉” and “...吃羊肉了”, I wrote the latter and it marked me incorrect.
Each language has its own logic and flow, use common logic to remember by comparision but please do not try to wrestle with it.
Why is 我 left out when saying "my friend"? Usually I would say 我和我朋友 for I and my friend.
Words are omitted when they can be implied.
我和朋友 can already mean I and my friends, because the friends that I have dinner with would naturally be my friends. We would like to specify only those combinations that are less predictable, like I and his friends, he and my friends, etc.
I would like to check something. I guess that this sentence would be wrong 我昨天晚饭和朋友吃了羊肉， As the subjet is 我和朋友, it cannot be splitted, and you have no other choice here than to start with 昨天晚饭 Right？
It is not wrong... if you allow me to be nitpicking then 昨天晚饭 is not perfect as a time element (昨天晚饭时 at the time of dinner yesterday) or as a demonstrative element (昨天晚饭上 on the dinner yesterday). But that's the same little problem that Duo's sentence already has. Especially when it is conversational, such things are well acceptable.
我和朋友 is not unsplittable and it depends only if you want "I" to be the subject, or the block "My friend and I" to be the subject. In the former case 和朋友 becomes an adverbial phrase to 吃. Another concern is when it has gone too far away in the sentence then we need to consider not omitting 我 from 我朋友.
I went to drink beer with him first at the time of dinner, and then I ate lamb with my friends.
BTW where the "l" in your name has gone? Just curious.
I guess, since wo (subject) start the sentence, it is not needed to repeat it in the second sentence. But it is implicitly there. 我昨天晚饭时先跟他去喝了啤酒，然后(我)和我朋友吃了羊肉。
I might be wrong, but I think you cannot split it this way because 和 has to be used in this fixed structure: Noun + 和 + Noun
Perhaps this would be correct: 我昨天晚饭跟朋友吃了羊肉
Please explain to me when it's OK to put the time period (in this case yesterday dinner) after the subject of the sentence and when it has to come at that the beginning? It's always a guessing game for me.
Also, I agree with other comments--why is了necessary here when the context clearly indicates past tense?
Time put at the very beginning provides the setting for the whole sentence.
At this time, this happens.
Time put before the verb (we don't make reference to the position of the subject), it complements the verb.
This happens at this time.
(In the case without a verb, the main attribute)