Questions & Answers about Иван, открой окно, пожалуйста.
Why does the verb end in -й here (открой)?
открой is the informal singular command form because we are talking to one person (Ivan) on a first-name basis. To make this form, we take the 'they' form of the verb (откроют - they open), drop the -ют, and because the remaining stem ends in a vowel (откро-), we add -й. If we were being formal or speaking to multiple people, we would add -йте to make откройте.
окно (window) is the direct object being opened. Why didn't its ending change?
You are right that окно is in the accusative case here since it receives the action. However, because it is a neuter, inanimate (non-living) noun, its accusative form looks exactly the same as its standard dictionary form.
Do I pronounce the й in пожалуйста?
No, the й is silent! It is pronounced as if it were written пожалуста. Because it is such a common polite word, Russian speakers say it quickly and drop that letter in everyday speech.
Why are there commas after Ivan and before please?
Russian punctuation rules are very strict about this: whenever you directly address someone by name, you must separate the name from the rest of the sentence with a comma. The word пожалуйста (please) must also always be separated from the rest of the sentence by commas.