To keep it simple, “that” can be deleted from a sentence most of the time. The notable exceptions include the ones below:
When “that” is part of a noun clause that acts as the subject of the sentence
- That he finishes his work is the most important.
- That the sky is extremely polluted should be noted.
When “that” links the relative clause to a noun (also called an essential clause, because the clause must stay in the sentence to describe what thing you are referring to)
- This is the brand that I was telling you about yesterday.
- That is the reason that we typically do not make exceptions.
Most other cases can have “that” or not. Whether you put it in does not change the meaning of the sentence, and people should be able to understand your meaning with or without it. The reason you might want to leave it in even if you do not have to is for clarity. When you have “that” in a sentence, it can signal that you are talking about something else, so it usually can provide just a bit more clarification.
Which vs. That! Is there a difference between Which and That?
