Learn Russian: Окно большое, и комната светлая. - The window is big, and the room is light.

In Russian, adjectives must match the gender of the noun they describe. Because окно (window) is a neuter noun (it ends in -о), the adjective takes the neuter ending -ое to become большое.

Why does светлая end in -ая instead of -ое?

Светлая describes комната (room). Since комната ends in -а, it is a feminine noun. Therefore, the adjective takes the standard feminine ending -ая.

Where is the verb "is" in this sentence?

Russian usually leaves out the verb "to be" in the present tense. To say "The window is big," you literally just put the noun and adjective together: "Window big" (Окно большое).

Is the comma before и (and) required?

Yes. In Russian, when you connect two complete mini-sentences with и, you must use a comma. Here, "the window is big" and "the room is light" are both complete thoughts with their own subjects, so the comma is mandatory.