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Eugene
They are worried about their boss replacing them.
Why does this sentence use ING "replacing" instead of "will replace"? 
Apr 30, 2020 2:52 AM
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“About” is a preposition, so it must be followed by a noun. The simplest way to turn any verb into a noun is to add -ing, thus forming the gerund. In theory, it should be “boss’s” (the genitive), but in reality, native speakers tend to use the object case. Compare the sentence with a regular common noun: “They are worried about their boss’s temper.” 

In the sentence you posted, I would not use the genitive - it just sounds weird. If you want to use a conjugated verb, replace “about” with “that”: “They are worried that their boss may (/ might / will / could) replace them.”

Edited to clarify which sentence I was referring to.
April 30, 2020
Eugene
Language Skills
English, Russian
Learning Language
English