For example, both of these sentences seem fine to me.

I'll do everything I can to make sure she's happy.

I'll do everything that I can to make sure that she's happy.

Is "that" really needed?

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    No, it's not. The complementizer that introduces tensed clauses and can be deleted from a complement (noun) clause like the object clause of make sure, but it is necessary if the complement is the subject of the sentence. I.e, that is necessary in That he arrived late is unfortunate, but not in It is unfortunate (that) he arrived late. – John Lawler Aug 16 '16 at 23:33
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    @JohnLawler I was confused for a moment at your comment, because I hadn't read the last part of the example sentence(s) carefully. In case you'd conversely skipped the first part, there's also a relativiser that in there (which is of course similarly necessary if no relative pronoun is present to take the subject slot). – Janus Bahs Jacquet Aug 16 '16 at 23:52
  • John, I have a question if you don't mind, sir. I'm new here and I've noticed that some people will answer the question in a comment instead of submitting an actual answer. Is there some rule of etiquette I've missed? I'm trying to understand why this is. Your answer here seems excellent, so I am wondering why you simply left a comment. Thanks for any help. – cliff900 Aug 16 '16 at 23:56
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    @cliff900 This is often a (site-)political rather than an English language issue. This question has appeared and been answered many times already, and 'answering' it yet again would send out a wrong signal to users, not encouraging them to do sensible checking before submitting a duplicate question. Giving the answer in a 'comment' is a way to help people while not encouraging misuse. // In John Lawler's case, I'm guessing he left politics way behind, and was very glad to do so. – Edwin Ashworth Aug 17 '16 at 0:14
  • @cliff900 Because I've answered this question several dozen times before here, and I get tired of fussing with the Official Answer stuff. Nobody ever checks them, they just keep asking the same questions. – John Lawler Aug 17 '16 at 0:16