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In this entry I want to look at some prepositions of time - in, for, till, by - which are often confused by learners of English. There are of course many more such prepositions, but I think it is better if we just focus on a few tricky ones.

IN - used with a period of time after which the speaker will do something. Japanese learners often try to use ‘after’, but this doesn’t sound natural.

• “I’ll call you in half an hour.” - ３０分後に電話するよ。
• “In 2 weeks I have to go to America on business.”
• “After 2 weeks I have to go to America on business.”

IN - used to indicate how long someone takes to completely finish something.

• “It was fascinating - I read the whole book in one day.” - すごく面白かった！１日（以内）に全部読んだよ。（本を読み終えたという意味です。）
• “I can probably get this work finished in a couple of hours.” - この仕事を二時間（以内）に済ませると思う。

IN - used in some fixed expressions.

• “Were you in time for the train?” - 電車は、間に合った？
• “I hope we get there in time.” - 間に合ったらいいけどなあ。

FOR - used to say how long someone spends doing something. The action is not necessarily completed.

not necessarily … - 必ずしも～ない

• “I’ll be in America for about three weeks.” - 三週間ぐらいアメリカに居ると思う。
• “I read the newspaper for 10 minutes on the train.” - 電車の中で十分間ぐらい新聞を読んでいました。（新聞を読み終えたかどうかが分からない。”I read the newspaper in 10 minutes“だったら、新聞を全部読んだという意味になります。）

TILL (or UNTIL) - means the same as the Japanese word まで.

• “I’ll be in America till Sunday.” - 日曜日まではアメリカにいるよ。
• “Wait here until I come back.” - 私が戻ってくるまで、ここで待っていて。

BY - means the same as the Japanese expression までに.

• “I should be back by Sunday.” - 日曜日までに帰ってくるはずですが。
• “I would like you to finish this work by tomorrow morning.” - 明日の朝までにこの仕事を済ませてもらいたい。

The most common confusion tends to arise with the distinction between ‘in’ and ‘for’, so let’s review the difference again. ‘IN’ often implies that something is completely finished, whereas ‘FOR’ does not. Compare these two sentences:

• “I’m going to read this book for 20 minutes.” - ２０分間この本を読むつもりだ。
• “I’m going to read this book in 20 minutes.” - ２０分間（以内）にこの本を（最初から最後まで）読み終えるつもりだ。

Actually, this last sentence is ambiguous. Depending on the situation, it could mean 「２０分後にこの本を読むつもりだ」, in other words that you are going to start reading the book 20 minutes from now.
ambiguous - あいまい
depending on - ～によって
in other words - 言い換えて

I hope you are making progress with your prepositions now!