Learn Russian: Окно круглое, а дверь большая. - The window is round, and the door is big.

In Russian, и is used to simply link things together, like items in a list. The word а is used to show a slight contrast or a shift in focus, often translating to "whereas" or "but". Because the sentence shifts the topic from describing the window to describing the door, а is the natural choice.

Where is the word for "is" in this sentence?

In the present tense, Russian drops the verb "to be" (is/are) completely. You just place the noun and its adjective side by side. So, Окно круглое literally translates as "Window round."

Why do the two adjectives end differently (круглое vs большая)?

Adjectives in Russian must match the gender of the noun they describe. Окно (window) is a neuter noun, so it takes the neuter adjective ending -ое. Дверь (door) is a feminine noun, so it takes the feminine ending -ая.

How do we know дверь is feminine? It doesn't end in -а or -я.

While it is true that most feminine nouns end in -а or -я, words that end in a soft sign (-ь) are a special group. They can be either masculine or feminine, and there is no strict rule to tell which is which by looking at them. You just have to memorize that дверь is feminine when you learn the vocabulary word!