Is sentence correct?

Do you mean factually correct (it may not be, I think) or grammatically acceptable? I would prefer to see an indefinite article before "sentence".

Careless people cross the road without looking.
Only careless people cross the road without looking.

No inversion there.

Hi Contrex

I mean whether Only starts the sentence is correctly phrased.

Many thanks.

Inversion is necessary if "only" starts the sentence.
Inversion is necessary if "only" begins the sentence.
Inversion is necessary if "only" is the first word in the sentence.
Inversion is necessary if the sentence starts with "only".
Inversion is necessary if the sentence begins with "only".
Inversion is necessary if the sentence commences with "only".

All OK

Only the lonely, dum dum dum dum , dum do we are.

Many thanks for your constant guidance, Contrex.

I don't understand about this "inversion" business.

Hi Contrex

If he studies hard, he will pass his examination.
Only if he studies hard will he pass his examination. (Inversion of 'he will' to 'will he'.)

This is what I meant.

That is correct. Slightly archaic though.

Thanks, Contrex.