Question

I am not sure. I will ____ (probably/certainly) not come for the party.​

Answer 1

Answer:

I am not sure. I will probably not come for the party.

We have used "probably" here because we are not sure.

Certainly means when we are a 100% sure.

Answer 2

Answer:

Probably is the answer.

Certainly is used for readily agreeable one like :

I will certainly come

or

We will certainly conduct it.

It is the clearly distinguishable fact. certainly used for affirmation and probably used for uncertainity.