"I already put that envelope in the mailbox."
Must the もう really appear before the ポスト? Can't it go before the 入りました?
Same question here. It seems more natural to me to put it before the verb but I may be wrong ?
Is there something wrong with this sentence? Or does there need to be a topic marker?
Duo's sentence is a statement about the envelop. It would answer the question, "What did you do with the envelop?" The presumptive topic of your sentence would be the speaker and there would be some contextual reason for saying what the speaker did as contrasted with what someone or anyone else did.
clears throat 'That mailbox already envelope in I put.' I get sentence structure is different in Japanese, but that's even more jumbled than it typically is. Not to mention the に is in a rather atypical location in regards to the term it refers to.
There are other possibilities but the structure in Duo's sentence makes "futou" the topic of the sentence (by marking it with "wa"). So, the sense of the sentence is "(As for) the envelop, already into the postbox (I) put." The "ni" does mark what it refers to.
I think you've misread the sentence. If I were to write it the way you did, it's "That envelope already mailbox in I put." The に is where it should be.
Though I do wonder why the もう goes in front of "mailbox" and not the verb. Is it an emphasis thing?
"Ni" is a postposition. It attaches to the word it marks which in this case is "posuto." The word plus "ni" when converted to English is "in the mailbox" not "mailbox in." Japanese has postpositions and English has prepositions. No misreading involved.
The sentence with "wo" instead of "wa" is grammatical but it implies an understood topic. That is, the sentence with "wa" tells you what happened to the envelop (which is the topic) while the sentence with "wo" is a comment implying that mailing the envelop has to do with a broader context (like things I've done today).