Если дезодорант закончился, я покупаю новый по дороге домой.

Because the idea is if/once the deodorant has run out. Russian often uses a past form here to show that the condition is already completed by the time of the main action. It is not necessarily about one past event; it can describe a usual situation.

For one specific future case, Russian would more often say Если дезодорант закончится, я куплю новый.

Покупаю is imperfective present, and here it expresses a habitual action: this is what I usually do in that situation. English does the same with I buy in sentences like If X happens, I buy Y.

If you said куплю, it would sound like a single future action, not a repeated habit.

Mostly if, but in sentences about repeated real-life situations it can feel close to when/whenever. The difference is that если keeps a conditional flavor: in the case that this happens.

If you used когда, the sentence would sound more like when/whenever this happens.

Закончился is the masculine singular past form of закончиться, which means to run out, to come to an end, or to be finished.

The -ся is part of the verb here. It does not literally mean oneself. Compare:

• закончить = to finish something
• закончиться = to finish / run out

So дезодорант закончился means the deodorant ran out.

Because it is the subject of закончился: the deodorant is the thing that has run out.

Russian often says this as the deodorant ran out, while English often prefers I ran out of deodorant. The structure is different, but the meaning matches.

Russian often leaves out a noun if it is obvious from context. So новый here means a new one, with дезодорант understood.

Both are possible:

• я покупаю новый
• я покупаю новый дезодорант

The shorter version is very natural.

Because the hidden noun is дезодорант, which is masculine inanimate.

After покупаю, the object is in the accusative case. For masculine inanimate nouns and adjectives, the accusative looks like the nominative, so:

• nominative: новый
• accusative: новый

If the noun were animate, the form would be different.

It means on the way home.

• по дороге = on the way / along the way
• домой = homeward / to home

So the whole phrase means that the speaker buys it while going home.

Because домой is used for movement toward home.

• домой = to home / homeward
• дома = at home
• к дому = toward the house/building, usually a more literal physical destination

In this sentence, the person is going home, so домой is the natural choice.

It is understandable and natural enough, but Russian speakers might choose slightly different versions depending on the nuance.

For a general repeated situation, you may also hear:

• Когда у меня заканчивается дезодорант, я покупаю новый по дороге домой.

For one specific future case:

• Если дезодорант закончится, я куплю новый по дороге домой.

The original sentence combines a completed condition with a habitual response, which is why it may feel a little unusual at first to an English speaker.