If you live like that constantly, youo will get chronic disease.

--> If you live constantly like that, you will get chronic disease. 

It is her chronic gatigue that make her sleepy.

--> Her persistent fatigue causes her to sleep. 

I think you should talk with your parents earnestly.

--> Good and correct sentence~ ^^ 

It's hard losing weight through only exercise.

--> It is difficult to lose weight alone through exercising.

He has been losing weight since last year.

--> Good and correct sentence~ ^^

She agreed as word, but she didn't sign on the document.

--> She agreed verbally, however, she didn't sign on the document. 

Stop using this medicine if you break out a rash.

--> If you develop a rash while taking this medication, discontinue use.

Food poisoning cause stomacheache.

--> Food poisoning can cause stomachache. 

Apparentely you still hurt, the doctor's diagnosis was fault.

--> You must still be in pain, as the doctor's diagnosis was incorrect.

Go to see a doctor and dont worsen your disease.

--> Visit a doctor to avoid worsening your condition.

Everyone having scallop had food poisoning.

--> Everyone who ate scallops got food poisoning. 

If the dish looks undercooked, don't have it.

--> If the food appears to be undercooked, avoid it.