Learn Russian: Мама моет окно. - Mum is washing the window.

The word окно (window) is the direct object receiving the action, so it is in the Accusative case. However, because it is an inanimate (non-living) neuter noun, its Accusative form looks exactly the same as its dictionary (Nominative) form. It doesn't change!

What is the dictionary form of the verb моет?

The dictionary form (infinitive) is мыть (to wash). This is a verb with a stem change, meaning the root vowel -ы- changes when the verb is conjugated. For a "she" subject like мама, it becomes моет.

Why is the first letter in окно pronounced like an "a"?

In Russian, when the letter о does not have the stress, it makes an "ah" sound. In the word окно, the stress falls on the final syllable, so it is pronounced ah-KNO.

How do we know it means "the window" and not "a window"?

Russian does not have articles like "a" or "the". You have to figure it out from context. Мама моет окно could technically translate to "Mum is washing a window", but in the context of household chores, "the window" is usually the most natural English translation.