Learn Russian: Я играю в теннис.

In Russian, when you talk about playing a sport or a game, you must use the verb играть (to play) followed by the preposition в and the name of the sport. You can think of it as saying "I play into tennis."

Does the word теннис change its ending here because of the preposition в?

Grammar-wise, it is in the Accusative case because it follows the preposition в to show the target of the action. However, because теннис is a masculine inanimate noun (it is an object, not a living being), its Accusative form looks exactly the same as its basic dictionary form. So its spelling doesn't actually change here!

How do I know играю means "I play"?

Russian verbs change their endings based on who is doing the action. The dictionary form of this verb is играть (to play). The -ю ending specifically means that "I" am doing the action. Because of this, you could even drop the Я (I) and just say Играю в теннис, and people would still know you mean "I play."

Does Я играю mean "I play" or "I am playing"?

It can mean both! Russian doesn't have a separate continuous tense like English does. Depending on the context, Я играю в теннис can mean "I play tennis" (in general, as a hobby) or "I am playing tennis" (right now).