"I came to Japan last week."
You don't use the に particle with times relative to now (e.g. Tomorrow, Yesterday, next year etc.).
I don't think Eric necessarily meant 'ni'. In other examples of referring to time, Duolingo uses the 'wa' particle.
は is used if the time expression is the topic of the sentence, the thing that is talked about. If the sentence was said in reply to the question "What did you do last week?", it would need a は, but if it was in reply to "Tell me something about yourself!", it should not have one (because "last week" is new information). Of course, Duoling never gives context, so both versions should probably be acceptable.
Oh, and another use for は is contrast -- if the sentence is in reply to the question "Terrible weather we had last month, wasn't it?", it should actually have a は even though "last week" is new information. But that is a special case.
I hear Japanese people use 今は、今日は all the time. If it's not necessary it shouldn't at least be wrong to include the 'wa' particle right?
先週日本に来ました - Last week I came to Japan
先週日本に行きました - Last week I went to Japan
And to further clarify: in the first case, you're still in Japan, and in the second case you're somewhere else. In English you can say "I came to Japan" while being somewhere else, if the listener is in Japan, but in Japanese you have to still be there yourself.
I used the correct vocabulary, kanji and the message said: the correct answer is: bla,bla,bla. The same answer I had provided !
Why can't I type the Japanese any more? Typing it was more fun and more challenging than merely selecting words from the word bank.
Why on earth is 先週日本へ来ました wrong? "友人のジョージがこの夏に日本へ来ます" is even an example sentence on https://context.reverso.net/translation/japanese-english/%E3%81%B8%E6%9D%A5