Question

If our party __ to be fun, I think our plan needs a lot of attention.

Answer 1

If our party needs to be fun, I think our plan needs a lot of attention.

or

If our party has to be fun, I think our plan needs a lot of attention.

or

If our party is to be fun, I think our plan needs a lot of attention.

Hope this helps,

ira.

Answer 2

If our party is going to be a fun, I think our plan needs a lot of attention.

Explanation:

I chose this option besides other one because it reflects toward future i.e. if our party is going to be a fun; I think our plan needs a lot of attention. The meaning of this sentence is that we need to have more plans if we want our party to be a fun.  

That’s the reason, I choose future tense. Although “will” also present in the given options but we never put "will" with "to" because It doesn’t make sense.

Hope it helped..............