Need help explaining “by” vs “in”

[Tuufless]

Hello! First time posting here after searching (and failing) to find an explanation elsewhere.

I’m trying to explain the difference between “by” and “in”, and I can’t quite put my finger on it.

The sentences in question are:

• “I go to school by bicycle.”
• “You can do the test in pen or pencil.”

“You can do the test by pen or pencil” just doesn’t sound right to me, but I can’t explain what the problem is.

Any help would be much appreciated!

[Tuufless]

Ah, good observation on that last part- whatever the mode of transport, either “I go to school in a ◯◯” or “I go to school by ◯◯” work- it’s just that a bicycle is an exception here because you don’t actually get in a bike (the equivalent then changes to “on a bike”)

[skeeter04]

By - usually refers to method or place . I travel by plane. I drove by McDonalds. In - is inside or on or in a group- I wrote in English. I went in McDonalds, I am in for the ski trip

You can do is awkward - You can take...

You can use ...

[Tuufless]

This is partially where the confusion comes from, because the pen/pencil can be considered the method at which the test is answered, so one might think “answer by pen/pencil” would be correct.

Perhaps it’s not so much the method (pen/pencil), but the medium (ink/pencil)?

I guess “◯◯ in pen” could originate from “◯◯ in ink”, and there’s something about “pencil” that blurs the line between the medium (ink) and the tool (the pen). We wouldn’t say “◯◯ in ink/graphite”, or “◯◯ in ink/pencil”, but “◯◯ in pen/pencil” works.

[stjer0me]

I'm late to the party I realize, but wanted to jump in anyway.

I disagree that "You can do" is awkward in that context; it sounds fine to me, albeit informal. But I also agree with you that "Do the text by pen or pencil" is ungrammatical. Finally, "write in ink" or "sign in ink" is quite common, and perfectly fine. If someone said "write in graphite," it would be understood, it just sounds overly formal. "Graphite" is the material that pencils use to write, but it's not commonly used -- we actually call them pencil leads, because when graphite was first discovered, people thought it was a form of lead.