1. "Один" (odin) - This is the most common way to say "one" or "a" when referring to a singular noun. It's used for both masculine and neuter nouns.
* Example: Один яблоко (one apple)
2. "Одна" (odna) - This is used for singular feminine nouns.
* Example: Одна книга (one book)
3. "Одни" (odni) - This is the plural form of "one" or "a" and is used for all genders.
* Example: Одни яблоки (some apples)
4. "Один из" (odin iz) - This translates to "one of" and is used to specify a single item from a group.
* Example: Один из этих яблок (one of these apples)
5. "Однажды" (odnazhdy) - This means "once" or "one time" and is used to indicate a single instance.
* Example: Однажды я видел медведя (Once I saw a bear)
6. "По одному" (po odnomu) - This translates to "one by one" or "one at a time."
* Example: Они идут по одному (They are going one by one)
7. "В один день" (v odin den) - This translates to "in one day" or "one day."
* Example: Он закончил работу в один день (He finished the work in one day)
It's important to choose the correct form of "a" based on the gender and number of the noun you are using.