Learn Hardcore Japanese: yama de kudamono wo tabemasita.

In this sentence, で marks the location where the action takes place. So 山で means “at/on the mountain.” It tells us you ate fruit while you were on the mountain.

Why is を used after 果物?

を is the object‐marking particle. It indicates that 果物 (fruit) is what you ate. Without を, the sentence wouldn’t clearly show the direct object of the verb 食べました.

Why isn’t there a subject like “I” or “we” in the sentence?

Japanese often omits the subject when it’s clear from context. Here, it’s understood that you (the speaker) ate fruit. In Japanese, you don’t need to say 私が unless you want to emphasize the subject.

What tense and level of politeness is 食べました?

食べました is the past tense, polite form of 食べる (“to eat”). It shows that the action happened already, and the speaker is using a polite style (the “‐ました” ending).

Could I use 山に果物を食べました instead of 山で?

No. に marks a point of arrival or existence, not the place of action. Saying sounds like “I ate the fruit to the mountain,” which is ungrammatical. For location of an action, you need .