Grammar question about “The third” and “third”:
In the following sentence: “The amount of population in A fell to the third place, replacing B.”
Why my grammar software suggests delete “the” before “third”? Is this just a bug or actually my mistake?
I thought there must be a “the” before ordinal number.

Question

Grammar question about “The third” and “third”:
In the following sentence: “The amount of population in A fell to the third place, replacing B.”
Why my grammar software suggests delete “the” before “third”? Is this just a bug or actually my mistake? 
I thought there must be a “the” before ordinal number.

Answer

In the example you gave, you do not use “the”. However, if it is used as an adjective, you use “the”:

“The first-place winner was John Smith.”

“He is definitely going to win the first-place prize.”

I am sure there is a grammatical rule about all of this, but I don’t know what it is. Hopefully a teacher can explain.

Answer

When you are dealing with expressions like “first-place” or “third-place,” you don’t need ‘the’ before it. Another example:

“They finished second-place in the race.”