I’ve been thinking about that. T-h-a-t. A handier word you’ll never find. Yet English speakers often omit it. That is left out. Suppressed, grammarians say. Implied.
Suppressing that doesn’t necessarily get you in trouble. Sometimes you can safely omit that when it follows a noun. Take shoes. Few misunderstand when you say the shoes you’re wearing instead of the shoes that you’re wearing.
Still, even following nouns, consider keeping your thats out in the open, especially if you write for those wonder workers we call translators or for people who struggle with English. Our language poses enough challenges when all the words are visible.
When it comes to verbs, though, don’t let that go without saying.