Learn Farsi: لطفاً پنجره را باز کن.

That symbol is called tanvin. It is a feature borrowed from Arabic that adds an "-an" sound to the end of a word, turning it into an adverb. Here, it turns the noun لطف (favor or kindness) into the adverb لطفاً (kindly or please).

Why do we need the word را after پنجره?

را is a very important marker in Farsi used to show a specific direct object. Because you are asking someone to open a specific window ("the" window) rather than just any window, you must place را directly after پنجره.

How does the verb باز کن translate to "open"?

Farsi uses many "compound verbs," which are made by combining a noun or adjective with a basic verb. "To open" is باز کردن (literally "to do open"). In this sentence, we see the command form: باز (open) paired with کن (the informal singular command for "do").

Why is there no word for "you" in this sentence?

In Farsi, you can usually drop subject pronouns like "you" (تو) because the verb itself shows who is doing the action. The verb form کن is strictly used for giving commands to one person informally, so the "you" is already built-in.