[Grammar] article confusing

[ShadeWe]

What's the difference between these two sentences?

This is the thing we need to worry about.
This is a thing we need to worry about.

I really want to know how native speakers see these sentences. In my opinion, considering the fact that there's a definition after the word, I need to say the. But as google says, there's an opportunity to say the a article before thing.

 

[SoothingDave]

Either is possible. Depending on whether you have mentioned the thing before and if there is more than one thing to worry about.

 

[jutfrank]

The first implies there's only one thing to worry about.

 

[ShadeWe]

Do you mean that I need to say "This is a thing we need to worry about" when there's more than one thing to worry about? Can there be 4 or 5 things?

 

[ShadeWe]

You answered when I was texting to SoothingDave

 

[jutfrank]

Yes, but it would be more natural to say something instead of a thing.

It doesn't matter how many things there are, just that it's not the only thing.

 

[andrewg927]

If there are two or more things, you need to change the structure to "These are 2 or 3 things we need to worry about".