sensei wa wakaranai mondai o shinsetsu ni setsumeishite kuremashita.

The teacher kindly explained the problem I didn’t understand.

Breakdown of sensei wa wakaranai mondai o shinsetsu ni setsumeishite kuremashita.

• は: topic particle
• を: direct object particle
• 先せん生せい: teacher

Questions & Answers about sensei wa wakaranai mondai o shinsetsu ni setsumeishite kuremashita.

Why is は used after 先生?

は marks 先生 as the topic of the sentence: as for the teacher...

It does not just identify who did the action; it frames the sentence as being about the teacher. In English, we usually would not translate は directly, but its effect is something like:

• As for the teacher, ...
• The teacher, ...

You might also see 先生が in other contexts, but は is very natural here when the teacher is already known or being presented as the topic.

Why is 分からない placed before 問題?

Because Japanese puts a modifying clause before the noun it modifies.

So:

• 分からない問題 = the problem(s) that I didn't understand = literally something like not-understood problem(s)

This is one of the most important differences from English. English uses a relative clause after the noun:

• the problems that I didn't understand

Japanese puts that whole idea before the noun:

• 分からない 問題

Why is there no word like that or which in 分からない問題?