Learn Hardcore Japanese: eki ni iku tuide ni, yobi no zyuudenki wo kaimasu. - I’ll buy a spare charger while I’m on my way to the station.

It means “while I’m at it / on the occasion of doing X, I’ll also do Y.” The trip to the station is the main purpose, and buying the charger is a secondary action you take because you’re already going.

Why is 行く in plain form instead of 行きます?

In Japanese, verbs inside subordinate clauses (like the part before ついでに) are normally in plain form, even when the main sentence is polite. So 駅に行くついでに、…買います is the standard mix.

What’s the grammar pattern for using ついでに?

With verbs: V-plain (nonpast/past) + ついでに (e.g., 駅に行くついでに, 銀行に行ったついでに).
With nouns: N のついでに (e.g., 買い物のついでに). With する-nouns, N のついでに is most natural (e.g., 出張のついでに). You’ll also see the verb version if you phrase it with a verb (e.g., 出張に行くついでに).

Can I say 駅へ行く instead of 駅に行く?

Yes. に emphasizes the destination (arrival), while へ highlights direction. Both are correct here, with only a slight nuance difference; へ can sound a bit more formal or “sign/post-like.”

Is the に after ついで required?

Yes. ついでに is a fixed expression meaning “on the occasion/while at it.” Leaving off に is ungrammatical in this usage.

Does ついでに mean I will buy the charger at the station?

Not necessarily. It simply means you’ll buy it on the occasion of going to the station. If you want to specify the place, add it: 駅で買います explicitly says “at the station.”

How would I say this in the past?

Use the past form in the subordinate clause and main verb: 駅に行ったついでに、予備の充電器を買いました。 Use nonpast when talking about a plan: 駅に行くついでに、…買います。

What’s the difference between ついでに, とき(に), and ながら?

ついでに: Do Y as a side task since you’re doing X anyway.
とき(に): Neutral “when/at the time of X, (do) Y” (no “side task” nuance).
ながら: Do X and Y simultaneously (e.g., 歩きながら).

Could I add も to emphasize “also”?

Yes. 予備の充電器も買います adds “also” to the object, emphasizing it’s in addition to something else you’re buying/doing. You can combine it with ついでに: 駅に行くついでに、予備の充電器も買います。

What are the readings and meanings of 予備 and 充電器?

予備 (よび): spare, backup, reserve.
充電器 (じゅうでんき): charger. Note: 予備バッテリー is a spare battery, not a charger.

Is 予備の充電器 the most natural phrasing? What about 予備充電器 or 予備用の充電器?

予備の充電器 is very natural. 予備用の充電器 emphasizes “intended for backup use” and is also common. 予備充電器 (compound without の) is possible but less common in everyday speech.

Why is there no subject like 私は?

Japanese often omits subjects when they’re clear from context. Here it’s understood as “I.” Add 私は only for contrast or emphasis.

Would 買ってきます be better than 買います?

Use 買ってきます if you mean “I’ll go buy it and bring it back.” Plain 買います just states you will buy it, without the “bring it here” nuance.

Is the comma after ついでに necessary?

It’s optional but helpful for readability. 駅に行くついでに、予備の充電器を買います。 and 駅に行くついでに予備の充電器を買います。 are both fine.

Can I use ついでに to make a request to someone else?

Yes. For example: 駅に行くついでに、予備の充電器を買ってきてください。 or more casually 買ってきてくれる？ It means “While you’re going anyway, please also buy it.”