Learn Russian: Окно комнаты большое.

In Russian, the present tense of the verb 'to be' (am, is, are) is almost always skipped. You simply put the subject and its description together, which literally translates to 'Window of the room big.'

Why does the word for room change from комната to комнаты?

Russian uses different word endings to show how words relate to each other. Changing the ending from -а to -ы puts the word in the Genitive case. This case is used to show possession or the word 'of' in English, so комнаты means 'of the room.'

Why does the adjective for big end in -ое (большое)?

Adjectives must change their endings to match the gender of the noun they describe. The word for window, окно, is a neuter noun because it ends in -o. To match it, the dictionary form of big (большой) takes the neuter ending -ое to become большое.

How do we know большое describes the window and not the room?

The adjective большое is in the neuter form, which proves it is matching the neuter noun окно (window). If the sentence meant that the room itself was big, the adjective would have to match the feminine noun комната and use the feminine ending -ая (большая).