Questions & Answers about Вчера мы были в парке.
Why do we use были here instead of был?
Были is the plural past tense form of the verb "to be". Since the subject is мы (we), which is plural, we must use были. You would use был for a masculine singular subject (like "he"), and была for a feminine singular subject (like "she").
Why is there an "е" at the end of парке?
To show location! When you say you are in or at a place using the preposition в (in), the noun needs to change its ending. This is called the Prepositional case. For a regular masculine noun like парк, this means adding an -е to the end, making it в парке.
In the present tense, we don't use "are" in Russian. Why do we need a verb here?
While Russian famously skips the "to be" verb in the present tense (so "We are in the park" is simply Мы в парке), you must use it in the past and future tenses. That's why были is required here to show that the action happened in the past.
Could I put вчера at the end of the sentence instead?
Yes, Russian word order is quite flexible. Мы были в парке вчера is perfectly correct and means the same thing. Starting the sentence with Вчера is just a very natural way to set the time frame right away.