"I put a cup on the table."
thank you so much for adding hiragana with your kanji! i cant read half of these comme ts because the kanji hasnt been presented yet.
I'm glad it helps! I try to keep them included for those who may not have learned them. So I'll be sure to try to keep them in my later posts. I've already learned more Japanese than Duolingo teaches, so I'm just going through and seeing who would need help with sentences.
With that said, I'm not exactly sure how I feel about Duolingo focusing so much on kana rather teaching kanji. Personally I feel that they should teach you the kanji from the beginning and teach you how to read it rather than only do kana. The more integrated kanji is and the more you use it, the easier it is to learn to read it. It's more intimidating in the beginning, but it's better for learning the language. And in my opinion, it actually makes reading easier since Japanese doesn't have spaces.
From someone who has been trying to learn Japanese from books, it's not a good idea to introduce kanji without being familiar with the underlying Japanese first. You use different parts of the brain learning conversational and written languages. It just ends up an overload of information.
Learning how to use it in a conversation like Duolingo does is much better. You can always learn the kanji later with furigana.
置いておきました would technically work, but the ～ておく construction is usually used to say you did something in preparation for later. So for example 昼ご飯を作っておきました would still translate as "I made lunch." but it has the implication of "I made lunch, so that I may eat it later." Without ～ておく it woud usually mean you just made it and are about to eat it, whereas using it means you'll eat it later.
So テーブルにカップを置いておきました would mean something like "I put a cup on the table for later." Sounds a bit weird, but maybe you're preparing the table for a dinner later? It has a close translation to "leave" rather than put. If you "leave" something in English, you're probably going to go back for it later.
That said, I am on board with it being accepted. There's a bunch of ～て forms that don't have a lot of good English translations. ～ておく being one of them.