Learn Farsi: ممکن است که مغز هنوز در مرحله‌ی خواب باشد. - It is possible that the brain is still in the sleep stage.

The meaning of hanuz depends on the verb. Because the verb at the end of the sentence (baashad) is affirmative (positive), hanuz means "still". If the verb were negative, hanuz would translate to "not yet".

Why does the sentence use baashad instead of ast for "is"?

The phrase momken ast ke (it is possible that) expresses possibility or doubt, which requires the verb that follows it to be in the subjunctive mood. The word baashad is the subjunctive form of ast (is).

Why is there a letter ye (ی) attached to the word marhale (مرحله)?

This is the ezafe connector, which links marhale (stage) to khaab (sleep) to mean "stage of sleep". Because marhale ends in a silent "h" (ه), the ezafe is written with a ye (ی) to connect the sounds smoothly, making it marhale-ye.

Do I always have to use ke (که) after momken ast?

In written and formal Farsi, ke (that) is standard after phrases like momken ast. However, in everyday spoken Farsi, it is very common to drop it, just like we often drop "that" in English when we say "It is possible the brain is still..."