Learn Russian: Теннис — хороший спорт.

In Russian, the present tense of the verb "to be" (like "is" or "are") is almost always left out. When you link two nouns together to say one thing is another, like теннис (tennis) and спорт (sport), you use a dash in writing to replace the missing "is".

Why does the adjective хороший end in -ий instead of the usual masculine ending -ый?

This happens because of a very common Russian spelling rule. After certain letters, including the "sh" sound ш, you are not allowed to write the letter ы. Instead, you must use и. So, the standard masculine ending -ый becomes -ий for this word.

Where is the word for "a" in the Russian sentence?

Russian does not have articles like "a", "an", or "the". The word спорт simply means "sport", but it naturally translates to "a sport" in English because of how the sentence is built.

Why are we using the masculine form of хороший?

In Russian, adjectives must match the gender of the noun they describe. The word спорт is a masculine noun because it ends in a hard consonant, so the adjective describing it must also take its masculine form.