The psychologist asked David

[Bassim]

I believe there is a mistake in my sentences, but I cannot point it exactly where. I feel that my first sentence sounds awkward, but I cannot correct it myself. Would you please correct my mistakes?

The psychologist asked David what he believed which event or a situation had changed him as a person. David replied that it must have been a premature death of his mother.

 

[teechar]

I believe there is a mistake in my sentences, but I cannot pinpoint it. [STRIKE]exactly where.[/STRIKE] I feel that my first sentence sounds awkward, but I cannot correct it myself. Would you please correct my mistakes?

The psychologist asked David what [STRIKE]he believed which[/STRIKE] event or situation he believed had changed him as a person. David replied that it must have been the premature death of his mother.

Consider saying "David told him/her" instead of "David replied that."

 

[Bassim]

teechar,

This sentence is typical for the problems I encounter while writing similar sentences in English. My inner voice tells them that I have made a mistake and that "he believed" should come behind "situation", but I am not sure where to put it and than make a mistake.

 

[teechar]

teechar,

This sentence is typical for the problems I encounter while writing similar sentences in English. My inner voice tells [STRIKE]them[/STRIKE] me that I have made a mistake and that "he believed" should come behind "situation", but I am not sure where to put it and [STRIKE]than make[/STRIKE] end up making a mistake.

In such situations, try writing the simple (uncomplicated) sentence first. So in this case we would leave out the "he believed". Then you can try inserting it where you suspect it might work, and evaluate the result.

Edit:
I also note that you're still mixing "then" and "than". Again, they sound quite different to the trained ear.

https://en.oxforddictionaries.com/definition/then
https://en.oxforddictionaries.com/definition/than

 

[GoesStation]

I also note that you're still mixing "then" and "than". Again, they sound quite different to the trained ear.

https://en.oxforddictionaries.com/definition/then
https://en.oxforddictionaries.com/definition/than

AmE speakers pronounce both words, when they're unstressed, with a schwa. ("Than" is, I think, nearly always unstressed.) They sound identical in that case. Do BrE speakers really distinguish them?

 

[teechar]

Whatever about the unstressed "than", I'm less likely to pronounce "then" with a schwa.

 

[jutfrank]

I'd go as far as saying that in BrE, then is never normally pronounced with a schwa.