Learn Farsi: من از جکوزی بدم می‌آید، مگر اینکه آب گرم باشد. - I dislike the jacuzzi unless the water is warm.

This is a classic experiencer verb construction! Instead of saying 'I dislike', Farsi phrases it as 'my bad comes from...' The structure is the adjective بد (bad) + the possessive enclitic -م (my) + the 3rd person singular verb می‌آید (comes). Because the literal grammatical subject is the feeling 'coming', the verb stays in the 3rd person singular, regardless of who is experiencing it.

If بدم already tells us the feeling belongs to me, why do we have من at the start of the sentence?

You don't strictly need it. The enclitic -م on بدم is entirely enough to show who is doing the disliking. Adding the independent pronoun من at the beginning is simply for emphasis or to clearly introduce the topic, similar to saying 'As for me, I dislike...'

Why is the preposition از (from) used before the jacuzzi?

Psychological verbs often use specific prepositions to link the feeling to the object causing it. With the compound verb بد آمدن (to dislike), the thing you dislike is always introduced by از. Literally, this translates to 'From the jacuzzi, my bad comes.'

Why does the sentence end with باشد instead of the normal present tense است?

The phrase مگر اینکه (unless) introduces a condition that might or might not happen. Like many conjunctions that express doubt, obligation, or conditions, it forces the verb that follows it into the subjunctive mood. is the present subjunctive form of the verb 'to be'.