Learn Russian: Извините, я не понимаю.

The -те ending makes this the formal or plural form of "excuse me." You use it when talking to a stranger, someone you do not know well, or a group of people. If you were talking to a friend or family member, you would use the informal version: Извини.

Where do I put the negative word не?

In Russian, не always goes immediately before the word it is making negative. Here, it goes right before the verb понимаю to mean "do not understand." Notice how there is no need for a helper verb like "do" in Russian!

Why does the verb понимаю end in -ю?

The -ю ending tells us that the action is being done by "I" (я). The basic dictionary form of the verb is понимать (to understand), but when you say "I understand," the ending changes to match the subject: я понимаю.

Do I have to include the word я?

No, you do not have to! Because the -ю ending on понимаю already makes it perfectly clear that "I" am the one doing the action, Russians often drop the pronoun and just say Извините, не понимаю. Both ways are completely natural.