IELTS Grammar: explain these little mistakes

In Wednesday's video presentation, I showed you a band 8 paragraph that contained four little mistakes. Here they are (in simplified form). Can you explain what is wrong in each case?

1. There are many reasons to believe why this is detrimental.
2. They have an impact on young adult's health.
3. The number of people doing physical exercises is falling.
4. By spending hours on television, we lead a sedentary lifestyle.

Comments

[Ahmet]

Believe to why

Adults'

Exercise not plural

Hours by spending on television

[Gumru]

1. There are many reasons (to prove) why this is detrimental.
2.They have an impact on young adults' health.
3.The number of people doing physical exercise is falling.
4. By spending hours in front of television we lead a sedentary lifestyle.

[VanessaC]

1. to believe that
2. adults'
3. exercise
4. with

[Chi]

1. There are many reasons why I believe that this is detrimental.
2. They have an impact on young adults' health.
3. The number of people doing physical exercise is falling
4. By spending hours watching TV, we lead a sedentary life.

[Jasmine]

https://books.google.com/ngrams/graph?content=many+reasons+to+believe+*&year_start=1900&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t2%3B%2Cmany%20reasons%20to%20believe%20%2A%3B%2Cc0%3B%2Cs0%3B%3Bmany%20reasons%20to%20believe%20that%3B%2Cc0%3B%3Bmany%20reasons%20to%20believe%20in%3B%2Cc0%3B%3Bmany%20reasons%20to%20believe%20it%3B%2Cc0%3B%3Bmany%20reasons%20to%20believe%20the%3B%2Cc0%3B%3Bmany%20reasons%20to%20believe%20this%3B%2Cc0%3B%3Bmany%20reasons%20to%20believe%20UFOs%3B%2Cc0%3B%3Bmany%20reasons%20to%20believe%20they%3B%2Cc0

https://www.google.com/search?lr=lang_en&tbs=lr%3Alang_1en&tbm=bks&ei=-V_-XbmkEJO_8QOX85-ICQ&q=%22impact+on+the+health+of+young+adults%22&oq=%22impact+on+the+health+of+young+adults%22&gs_l=psy-ab.3...4084.6372.0.6731.10.10.0.0.0.0.187.1492.0j9.9.0....0...1c.1.64.psy-ab..1.2.349...33i10k1.0.h8QSAfbeKms

https://books.google.com/ngrams/graph?content=+the+health+of+young+adults&year_start=1800&year_end=2000&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Cthe%20health%20of%20young%20adults%3B%2Cc0#t1%3B%2Cthe%20health%20of%20young%20adults%3B%2Cc0

https://www.google.com/search?biw=1229&bih=953&tbm=bks&ei=B2L-XarGL9Haz7sPwNG64A4&q=%22people+doing+physical+exercises%22&oq=%22people+doing+physical+exercises%22&gs_l=psy-ab.12...0.0.0.5500.0.0.0.0.0.0.0.0..0.0....0...1c..64.psy-ab..0.0.0....0.eStadya-XGo

https://www.google.com/search?biw=1229&bih=953&tbm=bks&ei=y2H-XarWFZvSz7sPvsqY8Ao&q=%22people+doing+physical+exercise%22&oq=%22people+doing+physical+exercise%22&gs_l=psy-ab.12...0.0.0.84484.0.0.0.0.0.0.0.0..0.0....0...1c..64.psy-ab..0.0.0....0.uRgV6CokcTo

https://www.google.com/search?tbm=bks&ei=NGP-XfCvA6DZz7sPuf-k-AI&q=%22spending+hours+watching+television%22&oq=%22spending+hours+watching+television%22&gs_l=psy-ab.3...6914.7905.0.8551.7.7.0.0.0.0.156.904.0j6.6.0....0...1c.1.64.psy-ab..1.0.0....0.Nujm6OfUPZk

[Dhiraj Rani]

1. There are many reasons to believe that why this is detrimental.
2. They have an impact on young adults' health.
3. The number of people who are doing physical exercise is falling
4. By spending hours watching TV, we lead a sedentary life.

[hang]

1. there are many reasons to believe that why this is detrimental.
2. they have an impact on young adults' health
3. the number of people who are doing physical exercise is falling
4. by spending hours on watching tv, we lead a sedentary life.

[Tsukasa]

1. There are many reasons why this is detrimental.
2. They have an impact on young adults' health.
3. The number of people doing physical exercise is falling.
4. By spending hours for television, we lead a sedentary lifestyle.

[saeid borhani]

1.There are many reasons why this is detrimental.
2. They have an impact on young adults' health.
3.The number of people doing physical exercise is falling.
4.By spending hours in front of a TV , we lead a sedentary lifestyle

[saeid borhani]

By spending hours in front of screens,
we lead a sedentary lifestyle.

[Sajan]

1.There are many reasons why this is detrimental.( repetition)
2.They have an impact on young adults' health.(plural)
3.The number of people doing physical exercise is falling.(singular)
3.By spending hours watching television, we lead a sedentary lifestyle

[Anuj jindal]

Thanks For This Valuable Information