I wouldn't say he's fat.

[yi-ing]

1-1 I wouldn't say he's fat.
1-2 I don't think he's fat.

2-1 I wouldn’t say I was the best student in high school. I failed almost every class!
2-2 I don't think I was the best student in high school. I failed almost every class!


What is the differences between each pairs?

 

[jutfrank]

It's hard to answer this without more context, or without hearing the intonation of speech. There are a few different ways that each sentence could be used.

As just one example, 1-1 could mean that you have just said he is fat and I want to express that I think you're being a bit harsh by using that particular word.

 

[Tdol]

1-1 could also be used to mean that you think he's fat, but are too polite to say it, except to your listener.