Learn Russian: Новый сериал начинается в 21:05, а потом будет реклама. - The new TV series begins at 21:05, and then there will be commercials.

You would say в двадцать один ноль пять. When reading digital time in Russian, you just read the hours as a regular number (двадцать один), say ноль for "zero", and then the minutes (пять). The preposition в (at) simply goes right before the numbers.

Why does the verb начинается end in -ся?

The ending -ся makes the verb reflexive. In Russian, if you use the regular verb начинает, it means someone is starting something (like "the director starts the show"). Because the TV series isn't starting something else—it is just starting on its own—you use the reflexive form начинается.

Why use the word а here instead of и for "and"?

While и simply joins two similar things together, а is used to show a slight shift in focus or a transition. Here, the speaker is moving the topic from the start of the TV series to what happens next (the commercials).

Why is реклама translated as plural "commercials" when the Russian word looks singular?