michi ni ochite ita tebukuro o hirotte, kouban ni motte ikimashita.

I picked up a glove that had fallen on the road and took it to the police box.

Breakdown of michi ni ochite ita tebukuro o hirotte, kouban ni motte ikimashita.

• に: destination particle
• を: direct object particle
• に: location particle
• 道みち: road

Questions & Answers about michi ni ochite ita tebukuro o hirotte, kouban ni motte ikimashita.

Why is 道 followed by に here? Why not で?

In 道に落ちていた, the particle に marks the place where something ended up or existed after falling.

• 落ちる means to fall
• something falls to / ends up on the road, so に fits naturally

So:

• 道に落ちていた = was lying on the road / had fallen onto the road

If you used で, it would sound more like the road is the place where an action happens. But 落ちる is not really an action you intentionally do in a location; it is a change that results in a new location, so に is more natural.

What does 落ちていた mean exactly?

落ちていた is made from:

• 落ちる = to fall
• 落ちている = has fallen and is in that state / is lying there
• 落ちていた = the past form of that, so was lying there / had fallen and was there

In this sentence, 落ちていた手袋 means:

• the glove(s) that had fallen and were lying there

This is a very common Japanese pattern:
verb + noun = the noun that ...

So:

• 道に落ちていた手袋 = the glove(s) that were lying on the road

Why is it 落ちていた手袋 instead of something like 落ちた手袋?