Learn Hardcore Japanese: watasi ha sensei no setumei wo rikaisimasita.

は is the topic marker. It tells the listener that you are talking about 私 (“I”). You’re essentially saying “As for me, (I) understood the teacher’s explanation.” It doesn’t mark the subject per se, but the theme of the sentence.

Why is の used between 先生 and 説明?

の indicates possession or attribution. 先生の説明 literally means “the teacher’s explanation.” It links the owner (先生) with what belongs to them (説明).

Why do we use を after 説明?

を marks the direct object of a transitive verb. Here, 理解する (“to understand”) takes 説明 as its object. So 説明を理解しました means “understood the explanation.”

Why is the verb 理解しました in the past polite form?

You’re reporting a completed action politely.
• 理 (understand) + 解 (untangle) = 理解 is a noun + する = “to understand.”
• しました is the past tense of する in polite speech, showing that the action is finished and that you’re speaking formally.

What’s the difference in nuance between 理解しました and 分かりました?

• 分かりました is more common and casual, focusing on the fact that you now “get it.”
• 理解しました is slightly more formal/academic, suggesting deeper comprehension or processing of information.

Can we say 先生の説明を分かりました instead of 理解しました?

No. 分かる is intransitive and doesn’t take を. You say 説明が分かりました (using が), not を分かりました. With 理解する, you keep を because it’s transitive.

Is it possible to omit 私は in the sentence?

Yes. Japanese often omits the topic when it’s clear from context. Just saying 先生の説明を理解しました still means “I understood the teacher’s explanation,” especially if you’re the one speaking.

Why is the word order 先生の説明を理解しました, rather than rearranging the particles?

Japanese generally follows a [Topic] [Object] [Verb] order.
• 私は (topic)
• 先生の説明を (object)
• 理解しました (verb)
You could swap topics (e.g. mention time or place first), but the object–verb order stays intact for clarity.