Learn Russian: Это окно.

Russian doesn't use a word for "is" in the present tense, and it doesn't have articles like "a" or "the". The word это works as a fixed pointer meaning "this is", so это окно gives you the full meaning without needing extra words.

Since окно is a neuter noun, did это change its ending to match it?

No, in this sentence это is a fixed pointer word meaning 'This is'. When you use it to introduce or point something out, it stays exactly as это no matter what gender the noun has. For example, 'This is a house' (a masculine noun) is still Это дом.

Why does the first letter of окно sound like an "a" when spoken?

This is due to a rule called vowel reduction. In Russian, the letter о only makes a clear "oh" sound when the voice stress lands on it. The stress in окно is on the last letter, so the unstressed о at the beginning relaxes into an "ah" sound.