Learn Hardcore Japanese: kaigai de tukatta ka douka ni kakawarazu, roominguryoukin wo kakuninsuru. - I check the roaming charge regardless of whether I used the phone overseas.

海外かいがい　で　使つかった　か　どうか　に　かかわらず、　ローミング料金りょうきん　を　確認かくにんする。kaigai de tsukatta ka douka ni kakawarazu, roominguryoukin o kakuninsuru.

I check the roaming charge regardless of whether I used the phone overseas.

Breakdown of kaigai de tsukatta ka douka ni kakawarazu, roominguryoukin o kakuninsuru.

をo

direct object particle

Questions & Answers about kaigai de tsukatta ka douka ni kakawarazu, roominguryoukin o kakuninsuru.

What does 海外で mean here, and why is the particle で used?

海外で means overseas / abroad as the place where something is used.

The particle で marks the location where an action happens. Since 使う means to use, 海外で使った means used it overseas.

海外で使う = use it abroad
海外に行く = go abroad

So で is used because this is the place of the action, not a destination.

What is the hidden object of 使った? Used what?

Japanese often leaves out things that are understood from context.

So 使った literally just means used or used it, but the sentence does not explicitly say what was used. In context, it is probably something like:

a phone
mobile data
roaming service
a SIM
the device/service being discussed

A natural English translation often has to add something like your phone or the service, even though Japanese does not say it directly.

Why is 使った in the past tense?