"do you think they will ... " vs "do you think if they will ..."

Question

I am still not fully clear on whether to use "do you think they will ... " or "do you think if they will ..."

E.g. "do you think they will be interested in this" or "do you think if they will be interested in this"

Thanks

Answer 1

We use if to introduce reported yes-no questions and questions with or.

Do you like me?(direct question)

I asked if he liked me.(indirect question)

Do you think they will be interested in this?
Is a direct question and you must use the first sentence.

Answer 2

I say:

do you think they will ...

You don't need an extra "if"

In your example:

do you think they will be interested in this

Would be correct, otherwise it sounds strange, you have to many extra words.