Learn Russian: Свидетель не помнит, когда он открыл сейф. - The witness doesn't remember when he opened the safe.

In Russian, you always put a comma before the question word (like когда, где, or что) when it connects the main sentence to an indirect question or statement. This is a strict punctuation rule you will see throughout this lesson.

Does the word order change when a question becomes an indirect question like this?

No! In English, we change the structure (e.g., the direct question "When did he open?" becomes the indirect "when he opened"). Russian is much easier: you just take the exact wording of the direct question (Когда он открыл сейф?) and glue it to the first half of the sentence with a comma.

Why does the sentence use the perfective verb открыл instead of the imperfective открывал?

The witness is trying to remember one specific, completed event in the past (the exact moment he successfully opened the safe). We use perfective verbs for single, finished actions with a result. If we used открывал, it would mean he was in the process of opening it, or that he used to open it regularly.

Since свидетель ends in a soft sign (-ь), how do we know its gender?

Свидетель (witness) is a masculine noun. Even though nouns ending in a soft sign can be either masculine or feminine, you can find clues in the rest of the sentence: the pronoun он (he) and the masculine past-tense verb открыл both point back to it.