Learn Polish: Mój szef jest bardzo zajęty, ale rozumiem go. - My boss is very busy, but I understand him.

go is a short, unstressed pronoun. We use it when the pronoun is just a normal part of the sentence. We only use the long form jego if we want to add strong emphasis, like 'I understand HIM (but not her)', or if it is the very first word of a sentence.

Can I change the word order and say '...ale go rozumiem'?

Yes, absolutely! Short pronouns like go usually prefer to sit right before or right after the verb. Both ale rozumiem go and ale go rozumiem are perfectly natural. Just remember that short pronouns can never be the very first word of a sentence.

How would this sentence change if my boss was a woman?

You would need to change a few words to match the feminine gender. 'My boss' becomes moja szefowa, 'busy' becomes zajęta, and the pronoun for 'her' is ją. So the sentence would be: Moja szefowa jest bardzo zajęta, ale rozumiem ją.

Why don't we need to use the word ja (I) before rozumiem?

In Polish, the verb ending already tells us who is doing the action. The -m at the end of rozumiem means 'I understand'. Adding ja isn't necessary unless you really want to emphasize that YOU are the one who understands, as opposed to someone else.