How do you say "I don't understand" in Japanese?

Quick use

Use わかりません (wakarimasen) when you need to clearly say that you do not understand.

At a public office, this is useful when the explanation is too fast, too technical, or about a procedure you do not know.

Chunk or pattern

This is a fixed phrase.

Learn the whole line first. It is safer to say this clearly than to pretend you understood.

Base form contrast

A useful nearby form is:

• わかります (wakarimasu) = I understand.
• わかりません (wakarimasen) = I do not understand.

This is enough for the beginning. You do not need a full grammar explanation here.

Practical note

This tells the other person the problem, but it does not tell them what to do next. A good follow-up is もういちどおねがいします (mou ichido onegaishimasu) or ゆっくりおねがいします (yukkuri onegaishimasu).