That

The problem with the word THAT for English learners is how often we use one of the possible meanings this word has. The  meaning in question is QUE which is used as a conjunction. In English using THAT as a conjunction is optional. Here are some examples:

I think THAT you're quite right.

She believed THAT you were guilty.

It will be a party THAT you´ll never forget.

In all these cases THAT is working as a conjunction (a conjunction is a word that connects two sentences). If you take it out the meaning remains the same so using THAT here is not essential.

I think you're quite right.
She believed you were guilty.
It will be a party you´ll never forget.
Now this is something we cannot do in Spanish. For us the use of THAT, which is QUE, is mandatory in this case and without it each sentence would sound awkward.
This only can be done is THAT is a conjunction but if it is working as the subject of sentence,  a ersonal pronoun, it is incorrect to eliminate THAT, e.g.
Rafael is the person THAT will be in power the next four years.
Medicine is that field of study THAT probably requires more study years.
Salt is that thing THAT should never be missing from any restaurant table.
In each of the cases above THAT is working as a personal pronoun, not a conjunction, and as such is also the subject of the dependent clause that follows it, therefore it cannot be eliminated.