Is this grammatically correct?

It sounds very strained, or like something from an old book. It is not a sentence that would be used in modern English.

The pattern seems to work better when the verb following is "be". For example, something like "I thought her to be a hard worker" is more usable, though still formal or literary-sounding.

Thank you so much for your kind reply and suggestion.

But what I want to know is whether it's grammatically correct or not. (Just for the grammar's sake) Is it?

Thank you so much.

For the purposes of natural modern English, you can consider it incorrect.

By the way, the passive form works:

She is thought to work hard.

Thank you again for the kind reply.

Sorry for troubling you, and I mean no offense at all. (really)

But you answer my questions with questions

shcho23It it's not suitable in modern English, was it in Old English?

Does that matter?

shcho23Why is the sentence structure grammatically incorrect?

Because native speakers do not accept it as correct. There is often no logical reason why one structured may be accepted when another is not.

shcho23It it's not suitable in modern English, was it in Old English?

Old English? You don't want to go there. Only a very small minority of people on this planet can read and understand Old English, let alone tell you what was grammatical in Old English a thousand or more years ago. Here's some Old English:

Nu sculon herian heofonrices Weard,

shcho23It it's not suitable in modern English, was it in Old English?

Note that there is a difference between "Old English" and "old English". Old English, or Anglo-Saxon, is, as CalfJim says, from a thousand and more years ago, and is mostly incomprehensible now except to specialists. "old English", which I expect is what you mean, could be English from any p

Sorry to all.. I went too far with Old English. I guess it was just a heat of the moment.

I sincerely apologize.

The guy who told me that the sentence was right is an English teacher (at a prominent college) here in my country.

And I absolutely disagree with him, so I wanted to know why it's wrong by asking it here.

Can the verb "think" not have 'to-infinitive',

shcho23The guy who told me that the sentence was right is an English teacher (at a prominent college) here in my country.And I absolutely disagree with him, so I wanted to know why it's wrong by asking it here.

"think" never comes up in any list of catenative verbs that follow that particular pattern.

I want him to work hard follows that