Learn Farsi: آن‌ها از لجنزار بدشان می‌آید.

In Farsi, "to dislike" is expressed using بد آمدن, which literally translates to "bad comes." It takes the preposition از to show what is disliked. You attach the possessive pronoun suffix, which in this case is شان for "their", to the word بد. This makes بدشان می‌آید literally mean "their bad comes."

Why is the verb می‌آید conjugated in the third-person singular when the sentence starts with آن‌ها (they)?

The actual grammatical subject of the verb می‌آید is the feeling of dislike (بد), which is singular. The word آن‌ها simply sets the topic of the sentence at the beginning, meaning "As for them." The connection back to "them" is handled by the pronoun suffix شان on the adjective, rather than by the verb ending.

How is the word لجنزار (swamp) formed?

It is made from لجن, meaning mud, sludge, or slime, and the location suffix زار. This suffix indicates a place abundant in whatever the root noun is. Therefore, a place full of mud becomes a swamp or bog. You will also see this suffix in words like چمنزار (meadow) or لاله‌زار (tulip field).

Could the suffix شان be attached somewhere else in this sentence?