if i want to say that something is going to be worse later if u do that ,can i say
-it's going to be worse next
-it's going to be worse later
-it's going to be worse afterwards

how can i say it?

and another one...if i want to say
-the only problem i have with going are my knees(in case of going to jog,for example)

if you use the first one, I'd be looking for another word on the end, like
"next year" or "next week", even "next time" .. I would not just end the phrase with "next"
Before you asked, you had already chosen " .... worse later " Emotion: stick out tongue. I think all three are Ok

The only problem I have with ( in ) exercises is my knees.
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 suzi's reply was promoted to an answer.
i think that maybe with "afterwards "it will work,dont u think?and about the knees,i dont know if it is with "are"or is,any advice will be useful
The first attempt is bad enough, it's going to be worse nextEmotion: smile ( next attempt )

To determine is or are has nothing to do with your knee or knees Emotion: stick out tongue. It depends on the singular or plural subject. Since you said ' The only problem ', then ' is ' is usedEmotion: smile
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just like that"it's going to be worse next"i mean after that it's going to be worse,"it's going to be worse afterwards or after"can i say it?i dont know if it's going to be worse next sounds good since i want to say just that sentence"it's going to be worse...."