the phase 'In my opinion'

Grammatically there is nothing wrong.

However, as "I think that" is very similar to "In my opinion", you are just repeating yourself. Strictly speaking, anything redundant in writing should be avoided.

You may say "in my opinion, he is wrong".

You may also say "I think that he is wrong".

But if you use both at the same time, it is redundant.

唔可以, 因為in my opinion, 係英文既意思度已經有i think 既意思, 所以如果再係后面加 i think 就會變得重複,多餘.....
it should be.........in my opinion,XXXXXXXXXXX (doing exercise is good for our health)

如果你真係想用i think....你可以用
Personally, i think.....................
或者真接用 i think..................

冇話得唔得，不過最好唔好，因為好多餘
↓
而我既意見呢，我諗．．

平時就冇乜所謂
但去到ｃｅ／ａｌ　就千其唔好-__-！